Sorry, a critical certificate used to verify add-ons expired.

As a temporary workaround, you can disable the add-on verification. I suggest setting it back after a real fix is announced.

(1) In a new tab, type or paste about:config in the address bar and submit that. Click the button promising to be careful or accepting the risk if presented.

(2) In the search box above the list, type or paste xpin and pause while the list is filtered

(3) Tap the xpinstall.signatures.required preference to trigger a Toggle button and tap Toggle to switch the value from true to false

Does that work on yours?

** EDIT **

Unfortunately, this creates the potential for a bad actor to serve Firefox an unsigned update, so it's definitely not a long-term fix. You can turn off extension auto-updating as a mitigation:

(4) In the search box above the list, type or paste autoup and pause while the list is filtered

(5) Tap the extensions.update.autoUpdateDefault preference to show a Toggle button, then tap Toggle to switch the value from true to false

Hopefully this won't be needed for very long.

As pointed out elsewhere, this settings change creates the potential for a bad actor to serve Firefox an unsigned update, so it's definitely not a long-term fix. You can turn off extension auto-updating as a mitigation. This would be added to the above:

(4) In the search box above the list, type or paste autoup and pause while the list is filtered

(5) Tap the extensions.update.autoUpdateDefault preference to show a Toggle button, then tap Toggle to switch the value from true to false

Hopefully this won't be needed for very long.