I've tried https://superuser.com/a/1363748/180573, but it seems the recovery.baklz4 doesn't contain any textarea items for Wikipedia edit forms.
I'm very disappointed; is there something that wikipedia does that prevents its textedits from being saved?
Is my data irrecoverably lost? Is there a way to preclude this from happening again? E.g., do I have to disable JavaScript for Wikipedia, or some such?
It appears that the Syntax highlighting feature of Wikipedia interferes with proper functioning of the sessionstore-backups/recovery.jsonlz4 in Firefox.
This feature can be enabled or disabled on the individual edit page, the last icon in front of "Advanced" drop-down menu.
I've verified that disabling the feature through the icon makes the whole textform appear within the sessionstore properly, whereas enabling it, makes the textarea mostly not saved (it seems like only the new stuff you type gets saved in that case, not the whole area like when the highlighting is not enabled).
EDIT: I think I was too fast; doesn't seem like the bigger pages get saved yet, but it does work on smaller pages…