I have repackaged Thunderbird for arm 32 bits from Debian and added two libraries older than the current one.
It works on my Diskio PI newly build with a Raspberry 4 8Go on which I have put Sarpi + Slackware current. I use KDE5/Plasma on it with the GDM desktop manager (SDDM is not working by default as Desiderius describe).
So for Thunderbird I have those SlackBuilds :
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https://github.com/BrunoLafleur/pbsl...ds/sbo/libffi6 (runtime lib for .so.6 with no includes or files which can have collision with the last version installed)
https://github.com/BrunoLafleur/pbsl.../sbo/libevent6 (runtime lib for .so.6 ...)
https://github.com/BrunoLafleur/pbsl...bo/thunderbird (version 78.8.0 - it seems they have more recent one I didn't test)
And it is working on KDE5/Plasma with the latest Slackware current.