Like every program, Firefox sometimes freezes and crashes, and some websites might not load correctly. These occasional glitches don't require your attention, but if the program repeatedly runs into a major problem, it might indicate trouble with your profile settings or an extension, or the program itself might need reinstalling. Full reinstallation is a last resort, however, and most solutions retain your browser customizations and bookmarks.
Initial Troubleshooting Steps
Before taking any elaborate measures, try rebooting your computer. Though a stereotypical answer to almost any computer problem, a reboot can actually fix all sorts of unusual problems by clearing everything out of the system's memory. If you're having trouble with a particular site, also try erasing your cache: Press "Ctrl-Shift-Del," set the time range to "Everything" and make sure "Cache" is checked in the Details section.
If these basic solutions don't help, the next step is to determine whether the problem lies in your Firefox profile, which contains your add-ons and settings, or in the software installation itself. As a test, open Firefox in Safe Mode: In Firefox 29 and above, click "?" in the Firefox menu and pick "Restart with Add-Ons Disabled." If you can't open Firefox at all, hold "Shift" as soon as you double-click the program's icon. If Firefox works in Safe Mode, you probably have a profile problem or trouble with plugins; if it doesn't work, you might need to reinstall the software.
Profile Problems -- Test Your Add-Ons
Buggy extensions and themes are a common cause of Firefox troubles. Even if each extension you use works independently, conflicts between extensions can lead to hard-to-diagnose issues. Assuming you can open Firefox, the first step to a fix is figuring out which, if any, of your add-ons is to blame.
If you recently added a new add-on, try disabling it first -- open the Firefox menu, click "Add-Ons," open either the "Extensions" or "Appearance" tab and press "Disable" on the relevant item. Otherwise, narrow down the problem by disabling add-ons, restarting Firefox and testing for the problem. To save time, disable half of the add-ons at once. If the problem remains, disable the other half instead. Re-enable a few at a time until the problem occurs, indicating the culprit, which you can then remove or research for a specific solution.
Profile Problems -- Refresh Firefox
If you can't solve your problem by disabling add-ons, or the program won't even open, refresh Firefox to reset the program to a working state, deleting add-ons, preferences and customizations but not erasing your bookmarks or history.
To refresh, visit the address "about:support" in Firefox and press "Refresh Firefox." If the browser won't open at all, hold "Shift" while launching Firefox, but instead of selecting to start in Safe Mode, press "Refresh Firefox" instead.
Software Problems -- Update Your Plugins
Plugins, not to be confused with extensions, refer to other programs that run inside Firefox, such as Adobe Flash and Java. Visit Mozilla's plugin status page to see if any of your plugins need updating. If so, update them to potentially solve the problem. If you want to uninstall any plugins, do so from the Programs and Features Control Panel, not from within Firefox.
Software Problems -- Reinstall Firefox
If Firefox can't open in Safe Mode or every other method fails to solve the problem, it's time to reinstall the program. Close Firefox and in any version of Windows since Vista. After the uninstaller finishes, delete the Firefox installation directory to remove any remaining files. Its default location is C:\Program Files (x86)\Mozilla Firefox. Reboot your computer,and then download Firefox using another browser and reinstall it.