useragent to be set to a firefox one when RFP is not enabled

As resist fingerprint is all or nothing, and at times one really doesn't want to enable RFP, then it's way more convenient to have the useragent set to Firefox rather than Librewolf. There are 2 options, it might be useful to set the useragent to the one used by RFP, even though RFP is disabled (that still helps), or if not possible, then the 2nd option is to use the corresponding Firefox version.

Any of those 2 options will do fine, specially for web sites misusing the useragent, or for Firefox extensions, which are harder to install when the useragent is not Firefox.

Thanks a lot !