I was using Windows 7. While browsing online, I clicked an email (i.e., mailto) link. I got this error:
Could not perform this operation because the default mail client is not properly installed.
A search indicated that there were only a few sources of information specific to the portable version of Thunderbird. The Mozilla page offered a reminder that I could set Thunderbird Portable to check whether it was the default email client. To do that, I had to go into Tools > Options > Advanced icon (not Advanced tab) > General icon. Mine wasn’t checked. That might have resolved it for me.
But I didn’t get that Mozilla page when I first searched, so instead I found another way that may be useful in some situations. Another Mozilla page pointed me to a registry location where there was supposed to be a mailto entry. Using the Regedit program available in Windows 7, I found that there was no such entry on my computer. I developed a .reg file that added that mailto entry. Once I ran that .reg file, the mailto entry was present in my registry, and clicking on the mailto link on a webpage did open a blank Thunderbird email message, ready to be written and sent. The .reg file read as follows:
Windows Registry Editor Version 5.00
[HKEY_CLASSES_ROOT\mailto] @="Thunderbird URL" "EditFlags"=dword:00000002 "FriendlyTypeName"="Thunderbird URL" "URL Protocol"=""
[HKEY_CLASSES_ROOT\mailto\shell\open\command] @="\"D:\\Miscellany\\ThunderbirdPortable\\ThunderbirdPortable.exe\" -osint -compose \"%1\""
To create and run that .reg file, all I had to do was to copy that code, paste it into Notepad, save it with a name like “Add Thunderbird Portable as Default Email Client.reg,” and then double-click on that .reg file in Windows Explorer to run it. Note that the path to ThunderbirdPortable.exe will depend on where you’ve put it and what you’ve named it.