Ook namens een vriendin:OS X 10.9.5Firefox 78.10.0esrWat is hier aan te doen?
Firefox 88 past niet meer op OS X 10.9.5.
Why has Firefox ended support for OS X 10.9, 10.10 and 10.11 users?While Apple does not have a public policy governing security updates for older OS X releases, their ongoing practice has been to support the most recent three releases. The last security update applicable to OS X 10.11 was made available in July 2018. Unsupported operating systems do not receive security updates, have known exploits and can be dangerous to use, which makes it difficult to maintain Firefox on those versions.
Updaten naar El Capitan of High Sierra is makkelijker, maar misschien heb je andere redenen om dat niet te willen.