I'm new to Subeta today so sorry if I don't make sense / have a hard time understanding you.

So..in stores I know you're supposed to be able to hover your mouse over clothing to preview it with your avi; a friend showed me. But I can't make that happen on any of my browsers.

What am I missing (plugins) / What is required to execute that?

I have Linux so once I know what I'm missing I have to ask a Linux website for help fixing my browser. : P

official shops run by Subeta's NPCs do not have the hover function. User shops do. Maybe this is your thing?

If you just want to see what an item looks like, you can swith either Wardrobe to "all clothing in Subeta". You can also go to the fansite Subetalodge and look the item up in their Item Database.

What actual things you need to add to your OS I'm not sure anyone in Help & Questions will be able to help you. You might try Profiles & Coding, or filing a Ticket in Help to get an official response.