Update hide-tabs-macos for Big Sur

[amomchilov]

Here's how the current styles look in Big Sur:

And here's the fixed:

Is it possible to detect MacOS 10.x (so it uses the old style), and only apply this for 11.x (Big Sur)?

[Timvde]

I have no idea :) Does this break anything on older MacOS?

[amomchilov]

Not broken, but the icons will be misplaced (according to the conventional layout pre-Big-Sur).

Actually, it looks better even on old versions. Here my test on macOS Catalina (10.15.7):

Firefox Out-of-the-box:

The current version of the mod:

My new version: