I got gmarks working on FF 38.0.5 (latest July 2015) and you can do this on any FF up to the latest, here's how:
1. go here and download & install Pale Moon Firefox variant:
http://www.howtogeek.com/108608/6-alternative-browsers-based-on-mozilla-firefox/* Don't create a new profile for Pale Moon*
2. Install Hide Caption Title Bar Plus add on
3. Intall gmarks add on
4. See the gmarks tab on top by placing the cursor on the upper left corner
5. Sign into your gmail account
6.
Gmarks will now work in Pale Moon and FF so long as you have Hide
Caption Title Bar Plus installed on FF. You can then either uninstall
Pale Moon or just don't open it. if you uninstall Pale Moon and gmarks
does not work then reinstall Pale Moon as above.
7. If have trouble
with this try opening the gmarks-1.0.5 unofficial_zynth_fx.xpi from FF
which is here. Just download it to your pc or notebook; Go to the F
upper left hand corner of FF > File > Open FIle > install
gmarks-1.0.5 unofficial_zynth_fx.xpi - but it should not be needed
I
discovered this quite by accident by trying Pale Moon to see if runs
faster than FF, it doesn't but as it si installed in the same profile as
FF it forces gmarks to work in FF. I had to reinstall my Win 7 64 and
it worked again :):)
Enjoy