How To Reinstall Firefox To Fix Speed And Stability Issues

Many tech lovers reinstall Windows from scratch every year or so because Windows becomes slower over time and is more likely to cause errors and crashes. Starting from scratch is a great way to speed up a tired Windows system – but does the same apply to Firefox?

If you have let Firefox upgrade itself over the top of the previous version for several years it is likely that your installation has also become more sluggish and prone to errors as bits of previous versions and old add-ons will have been left behind – just like Windows. If Firefox is slow or crashes a lot, consider it time to have a thorough clean up.

How Big An Issue Is It? As an example, we checked the default Firefox user profile on a computer upgraded over the years from Firefox 2 to Firefox 7:

  • Before – 37MB (171 files)
  • After – 16MB (39 files) – after removing Firefox 7 and reinstalling it from scratch (including all bookmarks/passwords and add-ons)

The huge decrease in profile size and cleaned up program folders resulted in a noticeable speed and stability improvement.

Why Not Just Create A New Firefox Profile? A new profile would still require bookmarks and passwords to be migrated across and preferences/add-ons to be reinstalled so doesn’t really save much time – you may as well clear out the Firefox program folders at the same time.

Completely Reinstall Firefox From Scratch – Warning – this is for advanced users and you must have a minimum of Firefox 3.5 installed!If you have an earlier version, update it to 3.6 or 7 now or you may lose key settings. If you have dozens of extensions and a heavily tweaked Firefox this guide may take you a long time – but the heaviest users will likely gain the most from it…

We have provided example backup procedures but if in any doubt, stop now. We recommend saving or printing these instructions.

1.  Download and save the latest version of Firefox from Mozilla here.

2.  Close Firefox and copy the whole of your Firefox profile folder(s) to a safe place on your computer – you will need it to import key settings (e.g. bookmarks and passwords) back into your new Firefox installation later on – the profile folder is called <profile ID>.default (where profile ID is a random 8 character name) and it can be found in the following place in your operating system drive (usually C: drive) – you may need to show hidden files to view these folders:

\Documents and Settings\<username>\Application Data\Mozilla\Firefox\Profiles in Windows XP or

\Users\<username>\AppData\Roaming\Mozilla\Firefox\Profiles in Vista and Windows 7

The profile folder contains sub-folders and files including a file called prefs.js – if it’s not there you’ve got the wrong folder!

Note: If you have previously created a new profile you must backup ALL the profile folders listed. And if you have more than one Windows user account you must backup the Firefox profile folder(s) for each and every Windows user account – each user has their own profile folder.

3.  As an additional safeguard, we recommend using Mozbackup to backup Firefox – again, do this for each Windows user account.

4.  Open Firefox and go to Tools \ Addons \ Extensions – make a note (or take screenshots) of all your installed extensions so you know which to reinstall later. Do this for each Windows user account.

Tip: some add-ons (e.g. Tab Mix Plus) provide options to Export and Import settings – if there is an option, export settings now so that you can import them back later. Again, do this for each Windows user account.

5.  Close Firefox. Last chance – are you sure you backed up all your profile folders, created a MozBackup and exported any add-on settings available – for each Windows user?

6.  Uninstall Firefox – when prompted to ‘Remove my Firefox personal data and customisations’ as shown below, Tick the box to select it then press the Uninstall button.

7.  Restart your computer

8.  Go to the following location and delete the whole Firefox folder if it is still present:

\Documents and Settings\<username>\Application Data\Mozilla in Windows XP or

\Users\<username>\AppData\Roaming\Mozilla in Vista and Windows 7

9.  Go to the following location and delete the whole Firefox folder if it is still present

\Documents and Settings\<username>\Local Settings\Application Data\Mozilla in Windows XP or

\Users\<username>\AppData\Local\Mozilla in Vista and Windows 7

10.  Now you can install Firefox to benefit from speed improvements:

Install Firefox by running the installation file you saved earlier – at the end of the installation Untick the box that says ‘Launch Firefox now’ and click the Finish button.

11.  Restart your computer and then open Firefox – this creates an initial new Firefox user profile. Now close it again so you can import settings.

12.  Import key settings from your old backed up profile into the new one. To do this, copy the following files from your backed up old profile into your new Firefox profile (overwriting those in the new profile) – the new profile will be in the same directory as in Step 2:

‘places.sqlite’ – stores the annotations, bookmarks, favorite icons, input history, keywords, and browsing history (a record of visited pages).

‘key3.db’ AND ‘signons.sqlite’ – store your saved passwords data.

‘cookies.sqlite’ AND ‘permissions.sqlite’ – store your saved cookies information.

‘mimeTypes.rdf’ – stores what action to perform when downloading certain types of files (save or open with a specified application).

‘formhistory.sqlite’ – stores saved entries for web forms and the Search bar.

‘persdict.dat’ – stores personal (custom) entries in the spell checker dictionary.

‘content-prefs.sqlite’ – stores site specific settings e.g. page zoom.

13.  Open Firefox and check that your bookmarks and saved website login passwords are present.

14.  Download and install any extensions (add-ons) you want – you can search from Tools \ Add-ons \ Get Add-ons.

15.  After installing all add-ons, import any settings that you previously saved (exported) from add-ons like Tab Mix Plus.

16.  Your user preference file will be new so you will need to reapply our tweaks to speed up Firefox if you had previously applied them.


You can check for yourself the size of your old (backed up) user profile and your new one to see how much of a saving it made. At the very least it will have removed both the clutter left over from previous upgrades and the remains of old/uninstalled add-ons which are commonly the cause of issues with Firefox.

Let us know if it made a difference for you!

5 thoughts on “How To Reinstall Firefox To Fix Speed And Stability Issues”

  1. Thankfully, I do a weekly backup on an external HDD so that, combined with resynchronizing my phone, got me back in business.

    I’m on Tbird 3.1.16 (had rolled back some time ago as more recent versions were not working well already – when even compatible! – with Lightning. So that’s not the source of the problem.

    It does seem illogical, but something strange did happen during the FF clean-out and reinstall. What the exact cause is remains a mystery to me.

    Thank you for responding though!

    • Strange, it still shouldn’t matter even though 3.1.16 is rather ancient and that means your Lightning is probably v1.0 which is old.

      I preferred the old Sunbird myself but glad you got it sorted anyway and FF is up to speed now.

  2. Thank you for the tutorial, that was very helpful and my FF feels much more fit now.

    However, this process also made the contents of my Lightening calendar in Thunderbird disappear (though I did not touch any Tbird, lightening or Sunbird files..). What do you suggest to solve that?

    Thanks in advance.

    • Lightning’s calendar data is stored in the file storage (.sdb) file in your Thunderbird profile folder – Firefox has a completely separate profile folder so reinstalling FF shouldn’t cause a problem in Lightning.

      The most likely reason for your problem may be this week’s upgrade to Thunderbird 10.0.2 – there are many reports on Mozilla forums of lost Lightning calendar data after upgrading and the two not working properly together e.g. see recent reviews at

      If you have a backup of your Lightning storage file you could restore that else you may need to ask Mozilla support forums for specific Thunderbird help

  3. thnaks for the guide i’ve been looking for an easy way to reinstall ff for a while as its been getting slow and keeps crashing on me

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