Feedbro is among a couple of unlimited RSS readers that I could find. The options menu provides a lot of customization for your feeds but ironically you can't automatically or easily arrange your folders alphabetically. Import and export features exist including Firefox sync option which I have not used yet because I don't know the sync size limit. Manually syncing is straightforward but it seems to accrete things instead of a snapshot in the way the file tree was arranged in the first place when saved as a OPML. Feedbro isn't great simply because no good free readers exists, it is overall well-done. However, it is not without flaws, the biggest one so far is that the extension becomes top-heavy if you add a lot of feeds and can impact browser and your overall computer performance. Those who are subscribed to a few or several feeds should not expect see large performance declines.