Try this, but don't permanently delete the files in case it doesn't help.

https://support.mozilla.org/bm/questions/1268486

Could you try a quick thing first, which is checking the page in Firefox's Safe Mode? Apparently, it is possible for badly behaved extensions to blank the Add-ons page, I guess so you can't dump them. But in Firefox's Safe Mode, extensions are disabled and powerless to prevent you from using the page. Here's how:

If Firefox is running: You can restart Firefox in Safe Mode using either:

• "3-bar" menu button > "?" Help button > Restart with Add-ons Disabled
• (menu bar) Help menu > Restart with Add-ons Disabled

and OK the restart.

If Firefox is not running: Hold down the Shift key when starting Firefox. (On Mac, hold down the option/alt key instead of the Shift key.)

Both scenarios: A small dialog should appear. Click "Start in Safe Mode" (not Refresh).

Can you get to the page? If so, I suggest being pretty ruthless in disabling all nonessential extensions.