Taken literally it means that when I use Firefox to upload a file to a customer's web site Mozilla is getting that file too, which does not seem likely. They could get a copy of the text I'm typing right now in Firefox or it means that the browser could do some local processing on those data. But if the results of that processing would stay local why would they ask the permission? It's not that emacs, vim, grep, sed, awk etc have to ask me the permission to use the information I'm inputting into them. So they are definitely sending information back home or they plan to do it.
The point becomes how to block any calls from Firefox to Mozilla. Note that don't have a Firefox account because I never trusted that the data in transit from them would stay private. I'm not logged in into Google as well. Maybe I have to finally install a Pi Hole and route all my traffic through it. Hopefully Blockada will take care of that for my Android devices.