The mozillazine link I posted above is from 2007 so not exactly current, but I have found the Thunderbird icons parked here:
/usr/lib/thunderbird/chrome/icons/default
I don’t know where the icon I have is being picked up from or where the proper TB icons should be placed to be used.
@swarfendor437 confirmed Evolution is mail client in the Core package.
Although I removed Evolution when I installed Thunderbird (via Software), I still have two Evolution folders:
/usr/lib/evolution
/usr/lib/evolution-data-server
Also synaptic indicates these Evolution left-overs from the software removal.
Is it safe to remove these two items using synaptic, or do they serve a purpose for the Z15Core package? (I need all the root space I can get)
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PS: If mods want to split this thread (as original Calendar problem is solved and now different topic) then feel free e.g. “Thunderbird wrong icons”