It could be that the installer ran long enough to corrupt your Firefox.

Let’s do a full clean re-install. I assume you want to reload 56? Or would you want to try the current? Download the program you want and save it. More instructions next post.

Install Older Version Of Firefox {web link} Be sure to read everything here.

If you still want to downgrade, look under; I still want to downgrade. Click the Directory of other versions and languages link. Look for the directory of the version that you want. But, remember that old versions may have security issues.

You should also check out Firefox; Extended Support Release {web link} ESR Notes System Requirements

Current Firefox; Download Firefox For All languages And Systems {web link}

Save the file. Then Close Firefox. +++++++++++++++++++++++++++++++++++++++++ Using your file browser, open the Programs Folder on your computer.

Windows: C:\Program Files C:\Program Files (x86)

Mac: Open the "Applications" folder. https://support.mozilla.org/en-US/kb/how-download-and-install-firefox-mac

Linux: Check your user manual. If you installed Firefox with the distro-based package manager, you should use the same way to uninstall it. See Install Firefox on Linux; https://support.mozilla.org/en-US/kb/install-firefox-linux

If you downloaded and installed the binary package from the Firefox download page, simply remove the folder Firefox in your home directory. http://www.mozilla.org/firefox#desktop ++++++++++++++++++++++++++++ Look for, and remove any Mozilla or Firefox program folders. Do not remove the Mozilla Thunderbird folder if there is one.

Do Not remove any profile folders.

After rebooting the computer, run a registry scanner if you have one. Then run the installer. +++++++++++++++++++++++++++ If there is a problem, start your Computer in safe mode and try again.

http://encyclopedia2.thefreedictionary.com/Linux+Safe+Mode This shows how to Start all Computers in Safe Mode; Free Online Encyclopedia

Sorry, everything worked before I saw your post. But I did almost the same thing (cleaning up the FF files), but only then I did next: I restored last successful recovery point of my Win7. I found folder that new Quantum created (old mozilla files or something like that). Then i replaced current profile to old one by copypasting. Everything is working fine. Also i deleted custom userchrome.css and chrome folder. I think the problem was because when you update from old FF, Quantum files cound't run some plugins or use older files and profiles settings and that causing crush. You guys should separate Quantum and old version. Make it so that they are two different lines of FF. Thanks!

That was very good work. Well done. Please flag your last post as Solved Problem so others will know.

The people who answer questions here, for the most part, are other Firefox users volunteering their time (like me), not Mozilla employees or Firefox developers.

If you want to leave feedback for Firefox developers, you can go to the Firefox Help menu and select Submit Feedback... or use this link. Your feedback gets collected by a team of people who read it and gather data about the most common issues.