Certain Firefox problems can be solved by performing a Clean reinstall. This means you remove Firefox program files and then reinstall Firefox. Please follow these steps:

Note: You might want to print these steps or view them in another browser.

1. Download the latest Desktop version of Firefox from http://www.mozilla.org and save the setup file to your computer.
2. After the download finishes, close all Firefox windows (click Exit from the Firefox or File menu).
3. Delete the Firefox installation folder, which is located in one of these locations, by default:
   • Windows:
     • C:\Program Files\Mozilla Firefox
     • C:\Program Files (x86)\Mozilla Firefox
   • Mac: Delete Firefox from the Applications folder.
   • Linux: If you installed Firefox with the distro-based package manager, you should use the same way to uninstall it - see Install Firefox on Linux. If you downloaded and installed the binary package from the Firefox download page, simply remove the folder firefox in your home directory.
4. Now, go ahead and reinstall Firefox:
   1. Double-click the downloaded installation file and go through the steps of the installation wizard.
   2. Once the wizard is finished, choose to directly open Firefox after clicking the Finish button.

More information about reinstalling Firefox can be found here.

WARNING: Do not run Firefox's uninstaller or use a third party remover as part of this process, because that could permanently delete your Firefox data, including but not limited to, extensions, cache, cookies, bookmarks, personal settings and saved passwords. These cannot be recovered unless they have been backed up to an external device!

Please report back to see if this helped you!

Thank you.

Try to repair the disk permissions:

• http://thexlab.com/faqs/repairprocess.html

Try to remove the Firefox plist file (org.mozilla.firefox.plist).

• http://www.macfixit.com/article.php?story=20060606082246983 How .plist files become corrupt and troubleshooting the results

Go to "~/Library/Preferences" and remove the plist file for Firefox (org.mozilla.firefox.plist)

You can check for problems caused by a corrupted localstore.rdf file.

• http://kb.mozillazine.org/Corrupt_localstore.rdf

Thanks for the suggestions - I solved it by backing up my profile folder, deleting Firefox and re-installed it. Then I moved the profile folder back into the default profile folder. The thing was that for FF 28 the profile folder was in ~/Users/username/Library/Application Support/Firefox but my previous profile was in ~/Library/Mozilla/Firefox/Profiles/<profile folder>...

So I guess the issue was Firefox 28/29 couldn't find the exact folder...

I am glad to hear that your problem has been resolved.

If you have not already, please mark this thread as solved by marking the solution.
This will help other users experiencing similar problems find help faster and more efficiently.

I hope you continue using our products and thank you for contacting Mozilla Support.

Have to open this again since the issue came back after I closed Firefox... Will try and do the clean reinstall as Daniel suggested.

The clean reinstall doesn't help, have the same issue. Will go and delete it as I did the first time (with Appcleaner) and move back the profile folder once I've reinstalled it.

After battling with the dumbest account creation system I have seen in a while, where you have to create a new question in order to register in order to reply to an existing post, I am finally able to respond to this issue. I was able to fix this problem by doing the following:

1. rename /Users/<username>/Library/Application Support/Firefox/profiles.ini to /Users/<username>/Library/Application Support/Firefox/profiles.ini.bak

2. start up firefox, a new profiles.ini will be created with a new default folder for this new profile

3. In Firefox select Bookmarks -> Show all bookmarks

4. Click the Star with something in it button. Tooltip says: Import and backup your bookmarks

5. Select Restore -> Choose File...

6. Pick the latest from your previous default profile folder (note: your json file will have a different date): /Users/<username>/Library/Application Support/Firefox/Profiles/<previousprofile>.default/bookmarkbackups/bookmarks-2014-04-08_1519.json

7. Once you have imported it, all your bookmarks will now be in your new profile.

8. You have to now go out and manually install all your extensions again from the Add-ons web site. If you think you can shortcut this step by copying your extensions folder and associated files to this new profile folder, you are mistaken and you will break this new profile, and have to start over again, don't do it, unless you enjoy repeating yourself.

From what I can tell, Firefox 28 has some issues with one or more of my extensions. I couldn't isolate which one, so the above procedure got me back and running again, minus the extensions.