If you cannot upgrade your OS, Firefox 78.15.0 ESR is your stopgap. However, for your own cybersecurity, it is highly recommended to look into upgrading your macOS version or considering a hardware upgrade soon.
In the panel, scroll to the Firefox Updates section. firefox download for mac 10105 upd
Many extensions still work on Firefox 78. For uBlock Origin, install from legacy collection – choose a version from 2021. If you cannot upgrade your OS, Firefox 78
: Install the latest root certificates manually, or use an extension like Let's Encrypt ISRG Root X1 to manually restore trust validation chains within your Firefox profile. Overcoming Modern Web Incompatibilities If you cannot upgrade your OS
Drag the Firefox icon directly into the Applications folder icon.