I have had a similar issue with my OS maps disappearing.
I use Firefox, and greasemonkey. I noticed there was a major update of Firefox a week or so ago, and I had not been able to access the OS maps since then, despite the usual restart/switch on and off etc.
I have just re-installed the plug-in as in step 14 in the instructions above, when I moved to step 15 the OS maps reappeared as if by magic
I wonder whether the firefox update had affected the plugin?
Hope that works for you