If Thunderbird is unable to remove an extension
You may try to remove (delete completely) a Thunderbird addon that you don’t need and you have already disabled, only to find out that the program is unable to delete it!
This is a problem you can fix easily; it is caused by folder permissions in your system profile’s Thunderbird folder. If this folder is configured as “read only” Thunderbird won’t delete its extensions.
Open Windows explorer and go to C:\Users\YourAccountName\AppData\Roaming\Thunderbird\
Select the “Profiles” subfolder and right click to open its properties.
Uncheck the “Read Only” option for this and all subfolders and files.