this box contains three squares, one contain a 1, the second the letter A and the third a down arrow. it appears and disappears quite quickly.
hello, can you try to replicate this behaviour when you launch firefox in safe mode once? if not, maybe an addon is interfering here...
Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems
How do i launch Firefox in safe mode?
this is described step by step in the help article i've linked to in my prior answer.
Can you attach a screenshot?
Use a compressed image type like PNG or JPG to save the screenshot.
attached is an image