Long long time FF/Navigator user at this point mostly by inertia. The biggest issue I have (forever) is FF is a memory pig that eventually becomes sluggish.
Though I do have a lot of tabs in four windows, I've set prefs so it only loads tabs when I click on them. So in reality I have a lot fewer tabs "active" on any given day. Yet I regulary see memory use grow from an initial 600MB to 4 or 5GB. FF seems not to properly release memory back, in particular after new tabs with video are closed. I might live with it were it not for the eventual slowdowns.