Using Thunderbird Portable and http links in emails are not opening in Firefox Portable or the browser you like to open them with? Here is a simple fix.
In Thunderbird Portable got to Tools and select Options from the list.
In this menu select the Advanced tab furthest to the right and go down to where it says Advanced Configuration. Click on Config Editor in that section, bottom right.
Promise Thunderbird you will be careful.
Type "external.http" into the search bar (without quotation marks of course!).
Double click on "network.protocol-handler.warn-external.http".
Double click on "network.protocol-handler.warn-external.https".
This will set both configuration options from False to True.
Close the Config Editor.
Go to an email inside Thunderbird with a http link and click it.
Now a window will open asking you what browser you want to use with http links. This works the same for https links as well.
Click on Browse and select "FirefoxPortable.exe" wherever you installed Firefox Portable.
Click OK and done.
Now when you click on links in emails they WILL open with Firefox Portable.