Add isFirefox and firefoxVersion to FeatureDetection

I don't think this needs to be mentioned in `CHANGES.md`, and a test couldn't do any more than verify that it returns true or false (and we don't have tests for the existing browser detection functions).

We needed this in National Map to enable `preserveDrawingBuffer` in Firefox due to this bug:
https://bugzilla.mozilla.org/show_bug.cgi?id=976173

We could add it elsewhere, of course, but I think it makes sense to go here.