Slimbrowser isn't Firefox browser? So how are you inferring that is Firefox browser issue?

Apparently, Slimbrowser doesn't have the July 2024 update that added support for the new extension signing certificate. The old certificate expired Friday.

• Related thread: https://support.mozilla.org/questions/1499473
• Related article: Update Firefox to prevent add-ons issues from root certificate expiration

You can try pressuring Slimbrowser to update, or switch to Firefox, or switch to another browser. If you want the most similar version of Firefox:

https://www.mozilla.org/firefox/all/desktop-esr/ => choose the 115 button when give the 128 or 115 option

It's not hard to add an ad blocker. This one is reputed to be the best:

https://addons.mozilla.org/firefox/addon/ublock-origin/

The userAgent you used to post this thread appears to be a Firefox 115.0 version on Windows 8.

To no have your Extensions be disabled and to be able to install new extensions you need at least Firefox 115.13.0esr (115.thirteen.0esr) or later like the current 115.21.0esr Release and will be updated till 115.27.0esr in August unless Mozilla extends updates again

The Fx 115.12.0esr (115.twelve.0esr)and older versions will have Extensions be disable and you cannot install new extensions.

Note Firefox 115 ESR is the only web browser still supporting Windows 7, 8, 8.1 with security/stability updates as all other browsers using Chrome/Chromium already dropped support back on Jan/Feb 2023.