When I'm reading web pages, I sometimes want to be able to report mistakes to the author, or fix them myself. However, it's awkward to maintain a separate list of mistakes, or bounce between Mozilla and my mail client for each one I spot; ideally I want to read through the entire document and then go back and deal with the mistakes.
The Mozilla Pedant Extension would give me a context menu item or gesture for "note problem here". Doing this over a piece of text would add a visible marker where the problem was, add it to a list, and light up an indicator at the bottom of the window.
Once I reach the bottom of the page, I'd click on the indicator to bring up the list; this could provide me with a to-do style list for fixing problems myself, or provide ready-formatted plaintext output showing each mistake with a bit of context. It could even tie in with Composer or with Wiki editing interfaces to allow me to go directly to a problem that needed fixing.