Have you ever wanted to send someone a link to a specific point on a lengthy webpage? Sometimes, you are lucky and the author included anchors throughout the page to specific topics (and even then that anchor might not be specific enough.) Or maybe you wish to bookmark something specific on a particular page w/o having to hunt for it later?
Add the ability to highlight a line of text on a webpage and create a link that'll take you/others directly to that line of (highlighted) text.
This wouldn't be difficult. Simply append the desired text to the page's URL (use the existing "#" anchor). Then from a (right-click) popup menu, offer the option to get the URL of that specific text. Then, when you bookmark/share the URL, it'll take you/them to exactly what you want to see (pre-highlighted.) Other browsers would simply ignore the superfluous link text.
It's a feature no other browser has that would make FF a much more attractive option.