ESR, see:
"Firefox Extended Support Release (ESR) is based on an official release of Firefox for desktop for use by organizations who need extended support for mass deployments. Unlike other release channels, ESRs are not updated with new features every six weeks. They are instead supported for more than a year, updating with major security or stability fixes." (1)
FF 17.0.11 ESR is from 2013 (2).
So, depending on your needs it "may" be suitable,
e.g. you need only a browser in your local intranet or so and it may need less resources than the v62
Remember, slitaz intends to be for low-usage hardware also
Security-wise I wouldn't use it for internet
TBH, I have never used v17 myself.
Erkan YILMAZ
http://people.slitaz.org/~eyilmaz/blog/
(1) https://developer.mozilla.org/en-US/docs/Mozilla/Firefox/Firefox_ESR
(2) https://wiki.mozilla.org/Releases/Old/2013