Start Firefox in Safe Mode {web link} by holding down the <Shift> (Mac=Options) key, and then starting Firefox.

A small dialog should appear. Click Start In Safe Mode (not Refresh). Is the problem still there?

yes. With starting in the save mode the problem remains: no webpage can be reached & most dialogscreens don't work.

Any issues residing in an existing profile might affect behavior for a newer version of Firefox, as profile data will be reused, even after reinstalling Firefox.

What happens in a clean profile? Please see the Profile Manager - Create, remove or switch Firefox profiles article for ways to launch the Profile Manager, or (temporarily) add <space>-p to your existing startup shortcut or a copy of it as described there. Do not make any modifications by adding add-ons. The only thing you could (or even should) do is disable hardware acceleration, as it’s enabled by default for new profiles.

Also, what’s the make and model of your CPU? If it’s Intel, please have a look at the Updating processor microcode support article - you may need a BIOS update.