After installing all of these and re-starting Firefox a number of times, I realised that the File -> New Window command was no longer working. Neither was the right-click Open Link in New Window command.
I wasn’t sure if this was a bug in Firefox or if one of the new extensions was causing a problem so I googled and couldn’t find anything that might indicate a problem with Firefox 1.06.
I then started disabling the new extensions one by one (and re-starting Firefox in between). Still no joy.
Then I remembered that I had updated the SearchStatus extension to 1.6. Disabling this extension resolved the problem and I am now able to open new windows once more (phew!).
Curiously there is no mention of this bug on the developer’s site – I reported it on the comment form on their site so hopefully we will see an update soon.