Improve firefox checker pattern

[terriko]

ENCODER=Mozilla([0-9]+.[0-9]+(.[0-9]+)?)",
This pattern exists in Mozilla Firefox checker too. >This could probably lead to false positives (we also >might want to take out the pattern from Firefox if this >pattern is present in Thunderbird)

Originally posted by @BreadGenie in #2429 (comment)

We probably need to fix the Firefox checker to avoid false positives with thunderbird.