Opening http links in Firefox

[Nigel Ridley]

I just downloaded and 'installed' Firefox 5 from the Firefox website.
I 'installed' it into my home directory: ~/firefox_5 and to run it I use the command:
~/firefox_5/firefox
I have even edited the main 'kicker' menu to reflect my new Firefox and it respects it - which is 
more than I can say for Thunderbird (about:config) and 'System Settings' > 'Default Applications' 
 > 'Web Browser'.

So how can I get Thunderbird and the 'gmail plasmoid' to open my nice new Firefox 5 instead of 
the system installed Firefox 3.6x?

Any help much appreciated,

Nigel