Add an option to restore the green background on the "Update available – download now"

### Is your feature request related to a problem?

The design of the green background on the "Update available – download now" item in the Applications menu was last seen in Firefox 88 and removed in the Firefox 89 Proton redesign.

### The solution I'd like

Add an `about:config` option to restore the green background on the "Update available – download now" item in the applications menu.

The only problem would be, that the design would conflict with the green "up arrow" icon, but I think, that it could be solved by changing the colour of the mentioned icon. But as I recreated that behaviour this problem isn't in that case.

### Screenshots or video

Original behaviour:

Current behaviour:

### Alternatives I've considered

_No response_

### Additional context

_No response_

No file or test is named. Start by locating the Applications menu styling and the existing about:config option patterns, then trace the update-available item and its green up-arrow icon. Done means the option restores the green background without an unresolved icon contrast conflict.

Written by the indexing model from the issue text.