the mentality of devs is that the hardware can take the bloat just give it some time and as far as I am concerned it's a cancer slowly eroding away at what software should be. quick, clean and efficient.
It's exactly the wrong move from a cost-effectiveness standpoint to put too much effort in supporting old hardware. The replacement cost for hardware is an order of magnitude less than the engineering cost of supporting that hardware. If we kept everything working on decade-old hardware, it would mean less features got developed, which meant people could do less with their computers. You personally might not mind, but society as a whole would make a loss on that deal.
Which is not to say the firefox devs don't care about performance. A lot of performance work went into FF 3.5, and memory-wise it's gotten much leaner than FF3. It does mean that if you're running hardware more than 5 years old, you won't be catered to, and for good reason.
If you're interested in the economic theory behind why it doesn't make sense to support old computers, I can highly recommend "Free", by Chris Anderson (the audiobook version is actually free itself).