Today I intended to install firefox & thunderbird on the mac. Everything went ok until I reached to the point where I could no longer open any of them, getting the message "a copy of thunderbird (or firefox) is already open", which is something I don't understand since I cannot see where it is open. The problems occurred well after I succeeded in importing data from both thunderbird & firefox I use on my PC.
That issue happens on PCs too when FFox or TBird cannot find the user profile data folder.
I'm assuming you had Win XP on the PC and the default folder for both TB and FF were used.
To migrate data from PC to Mac simply copy these two folder
Tbird
C:\Documents and Settings\User\Application Data\Thunderbird\Profiles\xyz.default
FFox
C:\Documents and Settings\User\Application Data\Mozilla\Firefox\Profiles\xyz.default
onto a USB stick USB disk or whatever.
User is your account user name
xyz is a random string
On the Mac side, create a folder named Mozilla inside your user Documents folder.
Inside Mozilla create two folder one for FFox and one for TBird
Name the first Firefox 😉 and the second Thunderbird 😉
Copy JUST THE CONTENT of xyz.default FF into Documents/Mozilla/Firefox
The same for TB into Documents/Mozilla/Thunderbird
Open Terminal and type for Firefox
/Applications/Firefox.app/Contents/MacOS/firefox-bin -p
and for Thunderbird
/Applications/Thunderbird.app/Contents/MacOS/thunderbird-bin -p
Those commands bring up the profile choser window.
You probably have a default profile. Remove it then select Create then continue and browse to the folder Documents/Mozilla/Firefox for FFox and Documents/Mozilla/Thunderbird for TBird
Execute Firefox and Thunderbird. You should see your e-mail and favourite and everything else you had on PC.