I assume because Firefox has been a ‘niche’ browser for a long time (pretty much for as long as Chrome as existed?); so the users tend to be a combination of die-hard fans and neurodivergents - two groups who tend to not do well with change of any sort (even cosmetic).
Don’t try to explain public roads, bridges, dams, water & sewerage, police, fire brigades or the military to her… she might just have an aneurysm.