Hello,

have you tried to open Firefox first and then click on help?

In Firefox go to Tools-Options-Advanced-General

Does is say that Firefox is the default browser? It shoud say at the bottom of this window. If not change it there.

Thunderbird uses the default browser. The one defined in your operation system.

Yes, I've tried that but it still opens a new IE window.

Firefox is the default browser.

I think I have fixed this by going to 'Control Panel', 'Programs & Features', 'Turn Windows Features On or Off' and unticking Internet Explorer.

it seems possible that there was some corruption in your system (registry) and once you uninstalled Internet Explorer, your system didn't have any choice but to insist on opening web pages in the default browser (in your case, FireFox)

There are numerous ways to 'screw up' defaults - both as DEFAULT PROGRAMS, and as ASSOCIATED FILE TYPES (Control Panel)

If messing with the control panel doesn't work, another idea is to: SET INTERNET EXPLORER as Default, check to see that it works as expected, reboot your machine, SET FIREFOX as Default; reboot. Win8 [ Control Panel \ All Control Panel Items \ Default Programs ]

Uninstalling may not be an ideal fix for those who use Internet Explorer occasionally --- as i do for "Print Selected" : I.E. allows you to "Print Selection" of text/images on a web page, and then VIEW (Print Preview) the selection and adjust the margins and 'Scale' so as to fit your selection in the correct SIZE and the correct NUMBER of pages. None of the plugins/extensions for FF seem to do this properly (in my experience).

Nonetheless (if you need Internet Explorer), it may be possible for users to do as "NIGH" has pointed out (uninstall Internet Explorer), reboot, and then immediately reinstall it. This may 'FIX' the OPEN IN [default] BROWSER issue (you might still have to go in and select FF as default), but you might have to accept other complications with the use of Internet Explorer - i.e. the different preferences that I.E. uses (that you had set before uninstalling) may have to be 're-selected' or 're-implemented'.

as always, 'your mileage may vary'