If you're using Mac OS X version 10.10.5 (Yosemite), you can still run an older but stable version of Firefox that is compatible with your system.
Thus, effectively means: Get the latest Firefox 78.x ESR update.
How to Download Firefox for Mac OS X 10.10.5 (Yosemite) If you’re still using a classic Mac running OS X 10.10.5 Yosemite
Are you looking to download or update Firefox on your Mac running macOS 10.10.5? You're in the right place! This guide will walk you through the simple process of getting the latest version of Firefox on your Mac.
If you want, I can draft a short webpage blurb, email, or step-by-step checklist tailored for end users or IT staff.
If your Mac hardware supports it, the best "update" isn't a new browser—it’s a new OS.
Mozilla no longer links old versions directly. Use the :
Are you open to trying ?