A seemingly common Firefox problem:

1. You want to check your list of Extensions (i.e: your browser’s installed add-ons). To do this you go to: top menu | Tools | Add-ons | about:addons
2. The top “Get Add-ons tab” of about:addons is active, and it wants to load. But can’t.
3. The “Loading…” button appears in the middle of this “Get Add-ons” page, but never completes “Loading…”. Firefox totally freezes.
4. You have to Alt+Ctrl+Del to force Firefox to close down.

This appears to be a local Internet service provider problem, because turning on a VPN (in the USA, in my case) before going to the “Get Add-ons” tab solves the problem. The page loads as it should.

I assume the problem arises because the ISP’s local cache of the relevant addons page hangs, and Firefox is unable to fallback gracefully from that failure.

One solution:

1. Load Firefox.
2. Turn on your VPN.
3. The about:addons / “Get Add-ons” tab should now load properly.
4. Switch down to the next tab, “Extensions”. (Which is what you wanted to view in the first place).
5. Turn off VPN and close the about:addons page. Close and restart Firefox.
6. Firefox should remember the last tab it was on for about:addons, this now being the “Extensions” tab. This tab, being seemingly local to your PC, should have no problem loading.

The “Get Add-ons” service being (temporarily) useless, you should instead use the Add-ons for Firefox Web page to find and install new add-ons.