It seems that Gmail released a little change just for Firefox 3.6+ that stops us rendering buttons correctly, so you will see ‘Search in Popup’ and ‘Send and’ as plain text.
In 3.0.20.4 I’ve fixed this. I also fixed a problem where GTDInbox could no longer detect adding/removing status labels to update the count of ToDo items (so the #todos would become inaccurate as you used Gmail).
As per usual, you can grab it right now from http://www.gtdinbox.com/gettingstarted.htm.

GTDInbox could not load (2 errors)
Some issues I’m finding now:
1. Labelling using the GTD buttons doesn’t work – it just makes the standard gmail ‘labels’ tab drop down.
2. Pre-labelling doesn’t work either
3. Clicking the magnifying glass next to a message in, for instance, s/action does not bring up that actual message, but rather the corresponding one (i.e. fourth in the list) from the inbox. I have not managed to clear my inbox to see what happens if it’s empty!
‘To Do’ tab and numbers seem to be working ok for me!
Things are a little peculiar since 3.0.20.4 installed this morning. Mostly GTDinbox doesn’t load at all (no reassuring appearance of “Compose to Self”). Re-loading Gmail occasionally pops up the GTDInbox side button, but clicking on the “+” does nothing.
Seems to be working well in Chrome, after disabling Right-side chat (too bad, liked that a lot). But, the Scheduled menu doesn’t fly out right, it just shows Scheduled, instead of Today, Overdue, etc. Even after disabling Labs.