Update:
I made an additional firefox profile, installed all the above
extensions, and all worked a-ok. Up to a point. Ubiquity suddenly
started behaving quirky and certain commands stopped working. I
unsubscribed them, and now I can't resubscribe to them because - as
above - the Subscribe button does nothing.
However, I managed to catch a chrome error with firebug when i clicked
the subscribe button:
window is not defined
file:///C:/Documents%20and%20Settings/Joel/Application%20Data/Mozilla/Firefox/Profiles/4qyq0q77.default/extensions/
ubiq...@labs.mozilla.com/modules/utils.js
Line 285
...and line 285 of utils.js:
window.openDialog('chrome://browser/content', '_blank',
'all,dialog=no',
urlString, null, null, postInputStream);