For a long time, Firefox has enjoyed a steady growth primarily at the expense of IE. One of the key reasons for switching, besides security, was sleekness - Firefox was known for running faster. If you were using Firefox in its early days, you can easily recall how it was a very lightweight browser.
As it has aged, it has certainly grown in complexity - but a core issues remains, that of memory usage. Now, one of Mozilla's board of directors has called the issue out, saying that memory concerns, particularly memory leaks or at least memory handling, are issues that need to be addressed:
"As Mozilla starts down the path to running in the mobile space we are spending time looking at memory pressure issues more closely,"
While the debate about whether or not the infamous browser is plagued with leaks has gone on for years, many concur that the longer you use it the slower it becomes. With Mozilla looking to take on Opera for mobile browser dominance, the longevity of a browser will come into play more. If a push into the mobile market encourages speed improvements on the desktop end, there's hardly a room for complaint. After all, if they continue to expect their marketshare to grow, they are going to need more and more points to sell it on, especially as IE7 brought Windows users many of the features IE6 lacked.