http://kb.mozillazine.org/Moving_your_profile_folder_-_Thunderbird
Starting from CLI allows more options, see:
thunderbird --help
After copying the profile from another computer, start with the profile manager and select the desired profile, after which normal starts will use it:
thunderbird --ProfileManager
I copied a profile from 4k resolution computer to a 2k one, now icons and text are huge. How to scale it back down? Don't see anything relevant-looking in the settings; I looked for "scale", "zoom" and "size".
In config editor, find property:
layout.css.devPixelsPerPx
then change its value from 2.0 to 1.0, or to -1.0 (which means match current X11 settings).
Make a fresh profile and examine prefs.js
diff until something looks promising.
$ thunderbird # Looks bad. Close it.
$ mv ~/.thunderbird ~/.thunderbird.bak
$ thunderbird # Close it. Now we have a fresh profile.
$ vimdiff ~/.thunderbird/newxxx.default/prefs.js ~/.thunderbird.bak/oldxxx.default/prefs.js
# Examine every difference until something looks promising. Value 2.0
# looks like a zoom value and it is refering to some pixel-to-pixel
# ratio - looks promising!
$ rm -rf ~/.thunderbird
$ mv ~/.thunderbird.bak ~/.thunderbird
$ thunderbird
# Open config editor, look for devPixelsPerPx and change to 1.0.
# Looks good!