Comment 7 for bug 1797945

Above is the direct Link to the German Version (DE).
Maybe you will have to chose your Language-Version from:
https://archive.mozilla.org/pub/calendar/lightning/candidates/6.2b6-candidates/build1/

Comment to the former Solution:
I did not have installed and did not try xul-ext-lightning.
You cannot find it in the new (Gnome-)Software. It only will show up in the old Ubuntu-Software-Center. However, there is a Comment, that this will only install the English Version (EN). You can also find this via Terminal > apt list xul-ext-lightning
If you want the english Version of Lightning, type: sudo apt install xul-ext-lightning