It would be good if you could give a little more context instead of just adding a subject with no other information.

What device are you using and can this be upgraded past Yosemite?

Lastly, as others mentioned, Firefox appears to be still supported but Yosemite has been unsupported since 2017 and is also not getting software updates so you should consider either moving to Linux if you wish to use existing legacy hardware or upgrading to a newer mac.

You might find this site helpful:

Mac OS Compatibility Guide

Which versions of macOS does your Mac support?

https://eshop.macsales.com/guides/Mac_OS_X_Compatibility

I was on Safari 8.0.8 and for some reason couldn't type anything in the body, I'm on my daughter's chromebook now.  For some reason I can't upgrade from Yosemite.  I formatted/erased the Macintosh HD as I was having trouble with the disk and it wouldn't boot.  Ended up with Mountain Lion instead of Yosemite.  I had to install Yosemite back.  Chrome started working again until I allowed an update.  I did have Firefox before the Disk wipe and OS reinstall.  Now I can't install any browser I try including Firefox, other versions of Chrome and Opera.

 Model Name:   iMac
  Model Identifier:     iMac13,1
  Processor Name:       Intel Core i5
  Processor Speed:      2.7 GHz
  Number of Processors: 1
  Total Number of Cores:        4
  L2 Cache (per Core):  256 KB
  L3 Cache:     6 MB
  Memory:       8 GB
  Boot ROM Version:     IM131.010A.B09
  SMC Version (system): 2.9f8
  Serial Number (system):       C17JW4TEDNCR
  Hardware UUID:        F62C807D-51D0-5EC7-A5CB-38E65A5D78F2
--

Found an old version that works for now.  Firefox 65.0.2, When I'm on Geekzone, it just gives me comfort that someone is there in the background while I sort these problems out even if I find my own solutions.  Thanks everyone!

Your model iMac13,1 should be able to go to a maximum of Catalina macOS 10.15.x and this is my own maximum for my iMac 14.2.

Anyway the latest version of Firefox 106.0.5 will run on Catalina.

To upgrade to Catalina macOS 10.15.x: https://apps.apple.com/us/app/macos-catalina/id1466841314?mt=12

 

1. Click the link to open the App Store page for that macOS. 
2. In the App Store, click the Get button to begin downloading the macOS installer.
3. After downloading to your Applications folder, the installer opens automatically. Follow the on screen instructions to install macOS.

PS: if you do not like Firefox tabs on top (above address bar) then Firefox is very customisable to change this and a bunch of other stuff. Here is what mine looks like after modification with css code:

You might need to upgrade to El Capitan (10.11) before you can go to Catalina.

This site might be useful:
How to Upgrade to macOS Catalina - TechCommuters