What's your computer system and Firefox version?

Let’s do a full clean re-install;

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) Note: Check Both Folders

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

Having the same problem here, with a slight variance.

68.0 -> 68.0.1 upgrade triggered Win10 (1809) BSOD; now cannot load my profile. "XML Parsing Error: undefined entity" as above.

Exact output:

XML Parsing Error: undefined entity Location: chrome://browser/content/browser.xul Line Number 50, Column 1:<window id="main-window" ^ </p>

NOTE: this forum is munging my output, probably some kind of anti-XSS protection. Attaching an image which shows the exact output.

I have tried the steps suggested above, did not make any difference. Note that if I rename my Profile folder I can launch Firefox into a new profile. I just can't seem to access my original profile. I have a lot of data there (bookmarks, history, tabs, creds, etc.) so I'm really hoping I'm not permanently locked out of it....