What is url? Firefox for mobile?

It is quite likely that you will regularly come ac across website that no longer support your Firefox 56 version.

Firefox 56 (Sep 28, 2017) is the last Firefox version that supported Legacy extensions.

Any special reason why you haven't updated during the past years ?

Your Windows 7 OS shouldn't have a problem with running the current release (102), but it will likely take quite some time to get used to current releases as a lot has changed during the past years. Best would be to update via "Help -> About Firefox" until you reach the current version to let migration code do it's thing and prevent losing data. You would at least need a Firefox 68 or 72 version.

I supose the best way is contact with GMail support.

I know what Google will suggest. ;-)