Going back to April of 2000 there has been Mozilla Bug #36351, which is a request to add JPEG 2000 support to Mozilla. There has been over 100 comments on this bug to date, but interestingly, Mark Shuttleworth made a comment this morning. The Canonical/Ubuntu leader simply said "We'd consider a patch for the Ubuntu builds of Firefox." This would be a patch that would add JPEG 2000 support to the popular Gecko-based web-browser.
Canonical picking up a JPEG 2000 patch for Ubuntu's Firefox would certainly be interesting and welcome, although it doesn't change the matter of JPEG 2000 image files largely being unused by a majority of Linux desktop users. With the feature freeze for Ubuntu 9.10 coming up on Thursday, this is work that would likely not come until at least Ubuntu 10.04 LTS, but there's always the possibility of having a PPA if this patch ends up materializing.