Mozilla Firebird is superb, but it hasn't stopped me from thinking about a few power-user features I'd like to see in my Web browser:
URL filters. I want to be able to write a small script handler that is invoked every time my browser loads a new page. The script would receive the URL of the page it was about to request and could run it through various text parsing routines (e.g. searches/replacements) before the page was requested. It would function, essentially, as a client-side mod_rewrite.
This could serve as a typo corrector (e.g. rewriting "google.co" to "google.com"), at the very least. For paranoid types, it could also be a NSFW-protector, which could rewrite a URL with a naughty word in it to a verification Web page on the local machine -- just in case you stumble upon something inappropriate at work. The dishonest folks in the crowd could whip up the one-line regular expression that converts a subscription-required Wall Street Journal article into a free one. I'm sure there are plenty of other applications.
HTML filters. Same as above, but the script would be able to alter the entirety of the document's HTML -- after it was downloaded but before it was rendered. The applications for this are vast. From a design/accessibility perspective, it'd be an exponentially more flexible type of user stylesheet, capable of altering layouts completely. It could also be an advanced adult-check filter that looks out for inappropriate words and serves up a warning/confirmation page. But most importantly, it'd introduce the ability to filter plain text to users' liking -- for example, replacing text in all caps with the lowercase equivalent, or automatically linking-up URLs and e-mail addresses that aren't already linked.
Clearly, there'd have to be a "revert to original HTML" button. But with such an intense level of customization, it'd be hard to go back.