For instance, in the settings page for Tab Session Manager, you can click an upload button to upload JSON session files from your device to the addon. I looked up if UBlock Origin Lite (the re-implementation of UBlock Origin with Manifest v3) supports custom filter lists and all the results said no it can’t due to the limitations of Manifest V3. OTOH I found a StackOverflow post and Google Groups discussion saying its possible with the filesystem API.
Your examples highlight two different questions:
Is a feature technically possible
Is Google going to implement a feature which is technically possible.
This is why you’re getting two different answers.
As far as FF’s implementation of MV3, I have no idea how they will implement it. The haven’t said much about MV3 besides that they’ll keep the adblocking features of MV2 alive.