fp_navigator and fp_useragent should automatically detect the Tor Browser version

fp_navigator and fp_useragent check for a Firefox version that is hardcoded in the test source code.

We should have some automatic way to detect the Tor Browser version we are testing and the Firefox version it is based on, so that we will not need to continuously update the source code (and maybe with we will be able to reuse the tests for multiple versions of Tor Browser).