Extension problems may arise after a Firefox update, not just after installing a new extension. In that case, it's impossible to know which extension is the culprit (due to its lack of compatibility with the new Firefox version). It may be enough to just update all the extensions in "Tools -> Add-ons -> Extensions -> Find Updates". Otherwise, the quickest solution to many Firefox problems is to check whether disabling all extensions helps.
1. Start Firefox in Safe Mode (not Windows Safe Mode). When the "Firefox Safe Mode" window appears, press the button, "Continue in Safe Mode". This will start the program with all extensions disabled and with the default theme.
2. If the problem disappears when all extensions are disabled, restart Firefox normally and disable about one half of the extensions. Open the Extensions Manager via "Tools -> Add-ons -> Extensions", then select each extension and click the "Disable" button. Restart Firefox.
3. If the problem is still gone when you restart Firefox, the buggy extension is among the ones you disabled. Note these down, and then enable about half of them.
4. Restart Firefox and keep narrowing down the number of suspects in the same way of always halving the number of suspects. If the "Firefox" problem comes back at any point in this detective process, you know that there is at least one buggy extension in the group you just enabled, but keep in mind that there could also be other buggy ones among the group you disabled. (So the only way of making sure that you find all buggy extensions is to only ignore those groups that did not cause any problems when enabled.)
You will probably want to uninstall any extensions found to be "buggy"; however, if the extension is important to you, you may want to see if reinstalling it (after first uninstalling it) solves the problem. Other alternatives are to keep it disabled and to regularly check for updates. You might also want to send the author a friendly note, and first check if s/he has a newer version on her/his site than where you got it from. Keep in mind that many extension authors are not professional programmers, and all do something else for a living than writing Firefox extensions. They may not even know that their extension is buggy, or they may need a reminder to find time to fix their extension