Hi, try holding down the Shift key when you start Firefox to start in Safe Mode. More - [[Troubleshoot Firefox issues using Safe Mode]]. If it works in Safe Mode, disable any theme or add-ons you have - Add-ons Manager (Ctrl+Shift+A) > Themes/Extensions and restart Firefox; then if it's OK, re-enable them one by one until you find the culprit. You may need to restart Firefox after enabling some add-ons. --------------------- If that doesn't help, try performing a ''Clean reinstall''. This means you remove your Firefox program files and then reinstall Firefox. This process does not remove your [[Profiles - Where Firefox stores your bookmarks, passwords and other user data|Firefox profile]] data (such as bookmarks and passwords), since that information is stored in a different location. To do a clean reinstall of Firefox, please follow these steps: '''Note:''' You might want to print these steps or view them in another browser. #Download the latest Desktop version of Firefox from [https://www.mozilla.org mozilla.org] (or choose the download for your operating system and language from [https://www.mozilla.org/firefox/all/ this page]) and save the setup file to your computer. #After the download finishes, close all Firefox windows (or open the Firefox menu [[Image:New Fx Menu]] and click the close button [[Image:Close 29]]). #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 [[Installing Firefox on Linux]]. If you downloaded and installed the binary package from the [http://www.mozilla.org/firefox#desktop Firefox download page], simply remove the folder ''firefox'' in your home directory. #Now, go ahead and reinstall Firefox: ##Double-click the downloaded installation file and go through the steps of the installation wizard. ##Once the wizard is finished, choose to directly open Firefox after clicking the Finish button. More information about reinstalling Firefox can be found [[Troubleshoot and diagnose Firefox problems#w_5-reinstall-firefox|here]]. <b>WARNING:</b> Do not use a third party uninstaller as part of this process. Doing so could permanently delete your [[Profiles|Firefox profile]] data, including but not limited to, extensions, cache, cookies, bookmarks, personal settings and saved passwords. <u>These cannot be easily recovered unless they have been backed up to an external device!</u> See [[Back up and restore information in Firefox profiles]]. <!-- Starting in Firefox 31, the Firefox uninstaller no longer lets you remove user profile data.Ref: Bug 432017 and https://support.mozilla.org/kb/uninstall-firefox-from-your-computer/discuss/5279 [Fx31] Windows uninstaller will no longer offer the option to remove personal data --> Please report back to say if this helped you! Thank you.

Thanks for your answer, unfortunatly, I was not able to open Firefox in Safe Mode. Selecting Firefox with or without pressing the Shift-key has no effect whatsoever. As I'm not able to open a Firefox-window, I cannot access the Firefox menu. I tried installing the latest version of Firefox, but I get the same message as when I tried to un-install: "Firefox must be closed to proceed with the (un)install, please close Firefox to continue." Are there other ways to close Firefox, when Firefox does not respond, and is invisible on my desktop and Task Manager?

The problem has been solved. After resetting Firefox appeared in Task Manager, thus I could close Firefox succesfully.