True. You probably can't just jump onto something like building (so hard and) secure Firefox or Chrome addons directly without any funding. Start with what you're thinking to build (talking about the protocol, the easy part). Once you're ready with it, keep improving it with the funding you receive in the shape of little commissions. Ultimately, you'll be able to build such add-ons for the protocol with the funding of total commissions.
Literally, "a lot of" crypto startups have become successful by following this path (starting with nothing, becoming everything). "Blockfolio" (mobile application) is an example of such success. IMO, crypto-industry is still lacking basic stuff, and start-ups (like the one you're planning) have so much potential!
Funding. As if every idea costs a couple of million in venture capital to get off the ground. The things with this idea is that The protocol part would be free. Webmasters just put the file on their websites. I could build the tipping app in a couple of weeks. Someone else could build the Chrome add on and another person the FF add on. It is open and extendable and could only encourage the spending of btc. the tricky part would be gettin people to know about it and use it.
As soon as you start saying "off chain", convert to USD, or anything like that, you are talking lots of money with little chance of actually doing anything.
This is the same principle as clicking a link in an email to something on Amazon and it opening the Amazon app at that item. Amazon has a file with the id of its app sitting on its website. When you click the link, the phone checks the website and only if that app is installed, will it open the link with it.