When I try to do something on gmail via their website, I am getting a warning at the top that the browser is not compatible.
Lastest version of FF, portable or local.
If I try anyway, all works very slow on the website.
Does that mean if one needs to use gmail for exmaple, the only way to access it is with chrome browser?
This is usually the result of an interfering extension. Disable them, clear cache, restart, and give it a shot, then enable one at a time to see which is messing it up.
Sometimes, the impossible can become possible, if you're awesome!