Extension requests expose Firefox's minor version and custom app name

Original report: https://github.com/mullvad/mullvad-browser/issues/386.

Basically, MB uses a custom user agent in requests started from extensions, such as:

Mozilla/5.0 (X11; Linux x86_64; rv:128.0) Gecko/20100101 MullvadBrowser/128.5.0

It doesn't happen on Tor Browser, because we don't customize it, but I think it'd be okay to include such a fix at the Base Browser level.

STR:

1. open the browser inspector
2. set it to multiprocess
3. go to the network tab
4. open uBO settings on filter lists
5. force the update of a list
6. check the user agent in the inspector