Hello Firefoxx81, it seems like this problem has already been spotted and fixed in Beta, so you should be receiving a fix in less than six weeks. Thanks for the bug report nevertheless. Cheers!

Firefoxx81 said

In the swedish version of Firefox 37.0.1 is the shortcommand for "hide firefox" is ⌘G and the shortcommand for "search again" is also ⌘G , it´s same shortcommand for both. In earlier versions of Firefox was the shortcommand for "hide firefox" ⌘H, why has it been changed? Now will no´t non of the two function works with shortcommands.

What you can do in the mean time is to go into keyboard settings and create a new shortcut specifically for Firefox. Choose Firefox as the app that the shortcut should apply to, enter "Göm Firefox" in the menu item text box, and simply hit ⌘H in the keyboard shortcut entry box. Works fine here until it gets fixed.