ISSUE! NOT WORKING ON FF 52.5.2esr, 52.5.3 esr !
Got exactly the same problem as described by starlight on Firefox 52.5.3 esr (also 52.5.2 esr) - no icons nowhere (also not in "settings" - neither “available services” nor “my services”), was not lucky enough to get the previous version (STARLIGHT, IF YOU READ IT, COULD YOU PLEASE MAKE THE VERSION 16.7.0 AVAILABLE? SAME REQUEST TO THE AUTHORS OF THE ADDON), but only got an .xpi file of a version 188.8.131.52.1 and am glad at least to have a working addon, but it's tricky to get the actual addresses of the services therein (just for example - there isn't even Google Drive link and the added button with it's address https://www.drive.google.com and Name 'GDrive' remains 'Custom Button 1' and shows errors etc.).
Do not want to give just one star etc., as i used this addon for years and was - and am - glad that it exists and do not want to ruin it, but it's impossible to write smth here without giving some amount of stars, so just took one star away from the 5, thinking, that if there were all 5, no one sees that I'm describing an issue.... A kind of a dilemma ;)