Firefox traditionally installed into Program Files. I know they are
working on background/silent updates, but I don't know if it will work
updating into Program Files. The entire reason FF updates have UAC
prompts now is because updating Program Files needs admin
permissions. Is one option for an upcoming FF update (which still
requires admin permissions) to install a SYSTEM level service/
scheduled tasks to auto-update? Then the SYSTEM level auto-update
task can run periodically by itself without explicitly asking for more
permissions.
But also, for BRAND NEW FF installs, they have moved away from Program
Files. Brand new FF installs default into a user's AppData directory
and which can also be updated without administrative blocks.