Drag the bookmarklet to your bookmarks toolbar (or wherever it is you want it), then go to any Flickr photo page, and click the bookmarklet and LOL
The implementation requiring nightlies follows canvas spec. The Firefox 3 implementation uses a non-standard "mozPathText" call and transformations.
LOLify! - Firefox 3.1a1pre (built on/after 06.17.2008)
LOLify! - Firefox 3
Special thanks to Erik Kastner of befuddlr for the original fragment of JS to grab the flickr image.
Also check out flolcatr, another project that uses the LOLcat meme idea and flickr images.