Can't start Thunderbird 45 and Firefox 46 simultaneously
Since the last automatic updates, I can't start Thunderbird and Firefox simultaneously. If I start Firefox first, and Thunderbird after that, I get an error message "Thunderbird is already running, but is not responding. The old must be closed to open a new window." Pressing "Close Thunderbird" does nothing. If I start Thunderbird first and Firefox after, that I get an error message "Firefox is already running, but is not responding. The old must be closed to open a new window." Pressing "Close Firefox" does nothing.
Chosen solution
Surprisingly, the profile folder for Firefox and Thunderbird is the same: "G:\mail". This is quite strange, as I use this folder as Thunderbird profile folder for years, but for Firefox, the profile folder was c:\user\user name\AppData\Roaming\Mozila\Firefox\Profiles\w5sjuxoc.default". I have no idea how did this happen (once, after the last update Firefox reset itself and I had to re-sync everything from the Firefox profile.) I created a new Firefox profile, copied the data and now the problem seems to be solved. Thank you for the help!
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What is the profile folder for both, Firefox and Thunderbird? Please provide the full path, including the name of the profile folder.
At the top right of the Thunderbird window, click the menu button 
, then select Help > Troubleshooting Information > Profile Folder > Open Folder.
This will open the profile in Windows Explorer. Should work the same way for Firefox.
Also do you have Thunderbird and Firefox installed into their own folders as done by default or installed into same folder?.
Chosen Solution
Surprisingly, the profile folder for Firefox and Thunderbird is the same: "G:\mail". This is quite strange, as I use this folder as Thunderbird profile folder for years, but for Firefox, the profile folder was c:\user\user name\AppData\Roaming\Mozila\Firefox\Profiles\w5sjuxoc.default". I have no idea how did this happen (once, after the last update Firefox reset itself and I had to re-sync everything from the Firefox profile.) I created a new Firefox profile, copied the data and now the problem seems to be solved. Thank you for the help!