While I find it very useful to search the page _titles_ using the awesome bar, I'd like the option to search the page _text_ of all those bookmarks. I imagine this to be implemented by either (1) the search bar providing a "Bookmarks" selection or (2) adding this feature to the awesome bar (perhaps when pressing a modifier key so that normal behaviour is retained).
Bug 342913 is an old tracking bug that covered this and other similar use cases. Implementing this as decribed would require that we start keeping and indexing copies of the actual content of every bookmark, which doesn't really seem like something we'd want to do in the near future (incremental user benefit compared to a normal web search seems minimal, and implementation costs are high).
A couple of interesting use cases: Local search would be a great addition to the Firefox "work offline" story. Imagine having all your favourite content, reference material, etc.. available via search while flying, in a remote area, or otherwise offline. Bookmarked content could be periodically (or on-demand) refreshed so that news headlines and other changing content are pulled locally.