Launching Firefox from Thunderbird adds backslashes to addresses
From MozillaZine Knowledge Base
Note that uninstalling and reinstalling Thunderbird does not reset the compatibility setting, the new Thunderbird version will "inherit" that setting. Also, be advised that Thunderbird 3.0 (and likely other Gecko 1.9 applications) will not start up in compatibility mode.
Bug 381922 – clicking any email link in TB2.0 gives exta \\ in firefox url box.