You said she’s running Lion, mac OS 10.7.
There are several releases after that which would still support her aging computer.
If you start the 
12 years ago, support for major version upgrades would run out 5 years ago.
I suspect her mac didn’t lose support for upgrades until either Yosemite or El Capitan.
Source: macOS - Wikipedia
OS upgrades are free.
Upgrading to one of those newer OS versions might facilitate a much new browser version.
Indeed:
" Firefox version 78 is the last supported Firefox version for Mac users of OS X 10.9 Mavericks , OS X 10.10 Yosemite and OS X 10.11 El Capitan."
Source: Firefox Mac OS X 10.9, 10.10 and 10.11 users move to Extended Support Release | Mozilla Support.
Suggesting that she should be able to upgrade to Firefox 78 (once her OS is upgraded to the latest supported version as above)