Please read Update Firefox to prevent add-ons issues from root certificate expiration

Thank you, Ty. Well, when I go to 'About Firefox' it says it's up to date. I'm wondering if the issue is that I'm using a very old OS (10.11.6 El Capitan, and can't upgrade) that isn't compatible with later Firefox versions.

@ Christine, good news: I think there is an easy fix for your extensions!

Since you are running an ESR (Extended Support Release) version of Firefox, you should be able to follow the following instructions to turn off the extension security check by setting the about:config preference "xpinstall.signatures.required" to FALSE. These instructions won't work in regular versions of Firefox, just the ESR (and "Nightly" testing) versions.

https://support.mozilla.org/en-US/kb/add-on-signing-in-firefox#w_what-are-my-options-if-i-want-to-use-an-unsigned-add-on-advanced-users

Also, I see that you're running Firefox ESR version 78.15.0esr (from 2021). There are newer versions of ESR that support Mac OS 10.11.6 El Capitan. The last version to support it is ESR 102 (from 2023). If you update from ESR 78 to 102, you will get more bug fixes for security and performance.

There are not as the old Firefox 78.15.0esr is indeed the last version to still support the old macOS 10.9, 10.10, 10.11

Firefox 79.0 to 115 ESR requires macOS 10.12, 10.13, 10.14 at minimum to run. Firefox 116.0 and later requires macOS 10.15 at minim to run.

macOS 10.12 is the minimum as you can see. https://www.mozilla.org/firefox/79.0/system-requirements/ https://www.mozilla.org/firefox/102.0/system-requirements/

Firefox Mac OS X 10.9, 10.10 and 10.11 users move to Extended Support Release (Firefox 78.15.0esr) https://support.mozilla.org/en-US/kb/firefox-mac-osx-users-esr

Thank you, Ty. Well, when I go to 'About Firefox' it says it's up to date. I'm wondering if the issue is that I'm using a very old OS (10.11.6 El Capitan, and can't upgrade) that isn't compatible with later Firefox versions.

Too bad you cannot update your macOS to 10.12 or later as you would then be able to use Firefox 115.ESR (115.13.0esr or later) currently at 115.21.0esr and your extensions would not be disabled and you can also install new extensions.

James, you are absolutely correct! Thanks for catching that. I was confusing macOS 10.11 with 11.0. I edited my earlier reply to remove the misleading comments about ESR 102.