Could you recommend a VPN useful especially for Firefox ?
Well, I use PIA, used to use nord... Both of these cost money, but work for your whole computer (and can usually be used for 5-10 devices).
There are also some browsers with one built in, such as Opera or TOR, this only works for this one browser (if you turn it on for some), but they are free. If you are doing browser downloads, it will work fine, you you are using a downloader, then you will need a full VPN, but then changing locations doesn't really work for Mega, have to restart the downloader too.