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wót D j dnja Maj 14, 2021 ·

Don't waste your time with TH v. 68. Doesn't work at all.

wót DaveRo (wuwijaŕ) Maj 16, 2021 ·

It's not clear if you mean Thunderbird 68 doesn't work, or just this addon.
I just installed TB 68.12.1 and installed the addon. It works fine.
Maybe you have other addons that interfere.
TB 68 is still used by people who rely on addons that are unavailable in 78.