None of the text is showing up in the cells of Google Sheets. Not sure if this is a Google or K-Meleon bug. I'm pretty sure this worked not long ago, but today I can't see any text, even if I start a new sheet from scratch. I'm using 75.1.
Sounds like something that needs permission for DOM storage (check in about:config page), perhaps you blocked that accidentally? Or blocked something else. Or wrong useragent? Cookies blocked?
It shows for me. But the first test input disappeared.
Also Google Sheets said that they don't support this version, but after that they do work, saving all input both from keyboard and clipboard.
The same time GS cannot open local xls-file reporting 'Upload error'.
When I've uploaded it through general Google Drive upload UI - it also cannot open in GS saying "Cannot convert file".