I've been stuck on Firefox 39 for years and nothing's happened to my system. ...
I can't think of many drawbacks. Some extensions will only work on newer versions of Firefox. Other than what I've mentioned, using older versions of Firefox isn't a big deal.
That's pretty much what I wanted to hear and so will continue to use Firefox ESR 45.9.0 and Thunderbird 45.8.0 — ± without qualms — until something significant bites the dust (similar to my hanging on to Mac OS 9* long past its viability).
The concerns raised by joemikeb I will take under advisement (ie, I won't do anything which might invoke such problems).
*
Side note for assembled multitudes' (almost certain) amusement:
I finally disabled and recycled my 2000 iMac DV SE Graphite running Mac OS 9.2.2 which was taking up more than 2 sq.ft of real estate on my desk ever since 2011 on the off-chance that it might be necessary to "re-commission" it just in case of a case (which of course never materialized). 
