I used my own HOWTO yesterday on my fiance's 2009 MiniMe installation and it wouldn't work. Then I have found 2 or 3 topics on this forum where this problem was marked as [SOLVED] and... Nada. Zip. Zero. Big Bobkas... SO I kept messing around and... I got it to work.
Its easy. Follow me.
Open Thunderbird and then click on
Edit >
Preferences
Now a new window popped up. Click on the
Advanced >
General >
Config Editor.
Click on
I WILL BE CAREFULL I PROMISE! When the warning pops up.
Now in the
Config Editor window find the
Filter field.
Copy this code:
network.protocol-handler.warn-external.http
and paste it into the
Filter field. Notice that You have
2 entries left in the field below. Double click on each of them to change their value to
TRUE (
this will make the entries embolden too - its ok nothing to worry about.)

Close the
Config Editor window. Go and find any e-mail containing a link to a www. If You don't have one... send one to Yourself... Click on that link. You will be presented with window which will look bit like this:

Now click on
Choose button.
You will be presented with a new window. This will look something like this:

Now click on the
File System in the pane on the left. This will give You new entries in the right pane of the window. Find
usr folder and click on it. Now find
bin folder and click on it. It may take a while to populate windows right pane this time as there are loads of files in this folder. For a moment Your window could look blank like this one here:

When the window is fully populated with files find the file
Firefox and click on it. Then click
OPEN button in the bottom right side of the window.

Now You will see that the window You have seen before has changed.

There is new entry called
firefox in it. Click on it. Now mark the tick next to the
Remember my choice for HTTP links and then click
OK button in the bottom right hand side of this window.
Your page should now open in Mozilla Firefox. Tadaaaaaaaaaaa. Check more links. Close reopen and retry Thunderbird / Firefox link connection. Hope it works as You expected.
Hope this helps to everyone who had problems with opening links from Thunderbird in Firefox. Remember to click on a https link too and configure browse like explained above.
Andy
P.S. I believe it will work in both 2009 and 2010 editions of PCLinuxOS (no matter what desktop environment) and if Thunderbird will be pointed to some other browser executable (like chromium, opera etc.) this could be used for setting other browsers as well (although I have never tested it so I can't be sure).