Extended Link Properties is a convenient Firefox add-on that displays file size when hovering over a link.
This extension is incredibly easy to use - simply add it to your Firefox browser and start enjoying the benefits. You can even decide whether or not you want to see the HTTP header by checking or unchecking a box in the extension's options.
If you're worried about compatibility, fear not - the Extended Link Properties extension works with Firefox versions 1.0 through 3.0. Overall, this is an incredibly useful tool for anyone who wants to gain a deeper understanding of the links they encounter online.
Version 1.2.5: N/A