From Proposal to Drop Mac OS X 10.3 (Panther) Support For Gecko 1.9 (thanks, MacSlash):
Dropping support for Panther would also free up engineering resources and allow us to take advantage of APIs that only became available on Tiger. We have made a huge number of great changes to our Mac OS X code for Gecko 1.9, but we still have a lot of work to do and we are already running short on time to deliver a product that works well on Tiger and Leopard. We will deliver a great product, our best yet - but every day counts from here on out. There is already a backlog of tricky Panther-only regressions, bugs, and performance issues[...] that I suspect will consume at least a few weeks of development time. Over the next 6 months, especially as QA ramps up for release, the number of serious Panther bugs will go up and thus require even more time. As for taking advantage of APIs that only became available on Tiger, I mean a couple of things. First of all there are some things Tiger can do that Panther can't. Thankfully that is a somewhat short list, but it is not insignificant.

I'd like to quote less, but would like everyone to read more. This is exactly the strange sort of conversation that drives me crazy, both when programming and as a user. Can it really be that 10.3 doesn't allow Firefox to work correctly? That OS X has bugs that simply have no workaround? 10.4 turns everything to gold, I suppose. Has Apple killed support for 10.3? 10.2?

I was wondering recently, spurred by a recent round of reader mail on about G3 support in 10.5 and my own continued daily use of OS 9 [sic], how long the Mac community tended to support an older OS. I figured it was one major release back. Still, today I run across Coda, which turns out to be 10.4+ only, and now hear this about Firefox.

Firefox as is will continue to be a good browser for a long time to come. Even Mozilla from 2002, iirc, remains a decent app for browsing in OS 9. Yet the fact that so much time can be saved by dropping support for 10.3 makes me wonder... well, it makes me wonder.

Here's a reply to the above doc from someone apparently working with Adium:

The point is that it's fairly old hardware, and those users are probably not the types to try an alternative browser -- they'll probably just stick with Safari.

To succeed on the Mac you need to be nimble and fashionable. Josh's work with native form controls and all the Cocoa port work in general has gone a long way, and I think with a nice theme and a few "hip" touches, we can make a big splash with Firefox 3. Dropping 10.3 will help us be more nimble.

"Nimble and fashionable"? I guess. Proof that if code doesn't rust, platforms apparently do. In brief, for me, if you're dropping support for an older OS version, you're helping push people to upgrade. That means you've become a salesman for, in this case, Apple, on some level. Make darned sure you think long and hard before you cut out users, regardless of their percentage of your perceived whole.

Perhaps it's unavoidable. Perhaps Firefox has moved beyond rendering web pages well with a minimal wrapper. Perhaps Firefox is too bloated to implement well xplat. Perhaps it is absolutely essential that it's able to "prevent a menu bar menu from opening when a user clicks on it with a context menu open already". Maybe that's mission critical for Firefox. I don't know. I don't know anymore. (apologies again to Mr. Mamet)