I'm going to assume this is on Windows. Before moving any data files around, back up your profile folder contents to reduce the potential for data loss. That the first optional step below. Could you try a "clean reinstall"? That involves hiding the current program folder and running a full installer. The installer should discover your Firefox data and start with your normal profile. If this doesn't start up better, the problem might be in your profile, so we would test creating a new profile as the next step. '''Clean Reinstall''' ''This assumes you run Firefox downloaded from Mozilla rather than from the Windows store. This might cause data loss with the Windows store version.'' This takes about 5 minutes. It is not necessary to uninstall Firefox, but you can if you like, saying No to any request about removing personal data. (A) [Optional] Back up your Firefox data. See: [[Back up and restore information in Firefox profiles]]. (B) Download a fresh installer for Firefox to a convenient location: https://www.mozilla.org/firefox/all/#product-desktop-release (C) Exit out of Firefox (if applicable). (D) Using Windows File Explorer (hold down the Windows key and press E to launch it), right-click > rename the program folder as follows (you might have one or both): Open C:\Program Files and look for Mozilla Firefox If you see it, right-click Mozilla Firefox, click Rename, and rename it to OldFirefox Open C:\Program Files (x86) and look for Mozilla Firefox If you see it, right-click Mozilla Firefox, click Rename, and rename it to OldFirefox (E) Run the installer you downloaded in step (B). It should automatically connect to your existing settings. Any improvement at your next startup?