The last least problematic FF version I have found is 15.0.1.
After 15.0.1, Flash conflicted, add-ons conflicted, PDF files would not download, etc.
I tried the ESR version. I tried the FF22-Portable.
Each one had it own set of conflicts with other things.
Mozilla rapid release insanity is creating a browser that used to be good but it now more buggy than Netscape 4.7 was.
Netscape? Wow, blast from the past. LOL!