I don't have documentation or anything authoritative here, but in general I would approach the topic of FF add-ons with some measure of caution where a terminal server is concerned. They will be going from the environment they were primarily designed for (single home user on a single PC) to one that is probably completely alien to them (multiple simultaneous users on a server OS). Depending on how the add-on was originally designed, they may or may not play nice in such an environment.
One approach might be to fire up filemon/regmon/whatever and then install/configure/run the add-on in a test environment. Keep a very close eye on what it does and how it does it. Only if you're satisfied that it keeps within the bounds of the currently logged on user, should you then provisionally OK it.
I would also advise the users that a TS is a shared service, and as such it is possible for the actions of one user to compromise or otherwise affect availability of some or all of the services on it for some or all of the other users. (A specific relevant example would be an FF add-on that leaks memory over time - it might be small enough to not even get noticed on a single user PC that's rebooted regularly, but on a ~100 user TS that's being used constantly it will build up quite fast indeed. Similarly one that doesn't leak but does have high RAM requirements would be mostly fine on a PC, but multiply that RAM requirement by 100 and what have you got? Big ouchies.) As a consequence, anything that you do green-light is subject to be withdrawn again at minimal notice.