I had a similar situation with one addon , i.e. Quick Restart. Quick Restart is marked as incompatible in both FF13, and the separately installed FF13 beta. FF13 which was formerly 12, does not have a string :extention.checkCompatibility13.0, user set, boolean, false, which would allowed it to work, but, rather a default, which does not seem to work. I have no idea how to change the strings, or delete them in about:config.
I have had no problem, getting addons to work, till this new v.13.
UserAgent: Mozilla/5.0 (Windows NT 5.1; rv:13.0) Gecko/20100101 Firefox/13.0
A minority may be right, and a majority is always wrong
~ Henrik Ibsen
WinXP, SP3, 512 MB, SM2.17.1, FF21, TB17.O.5, IE8.0, Google Chrome 26, Ghostwall , Kingsoft5.6, JRE1.7_21.