i remember something like that when i switched too- two possibilities. be sure you've got most current (but not Beta version) Firefox. and then there are settings that might help.
i only have to sign in once now 90% of the time- Particularly if i chose the SAVE SESSION windows when i closed Firefox.
at worst it prompts a sign in screen but has allowed me to have all the sing in info preentered. so it's a second one key enter/return hit.
i like firefox's controls a lot - the settings (e.g. re cookies) are worth setting to me. I'm a mac fan myself
Hope and Healing, An