Maybe I'm asking to much, but this seems totally unacceptable to me -- both things seem totally unacceptable to me. And while it may still be just true, Firefox suffers worse from this problem than any OS, software or other browser that I use.
Mozilla can do nothing about you using Firebug, yet still gets blamed for the slowness.
I was responding to YOUR stipulation that browsing experience degrades the more you use Firefox. IF it does, that seems like emergency problem #1 to fix if Firefox wants to stop hemmorrhaging users. But then, I'm speaking completely out of turn, since I haven't been a Firefox user for years.
EDIT: ...since I haven't used Firefox as my primary browser in years. I use it daily; not as a "power user."
This is how things go in real-world. I remember reading about how much work went into each new version of Windows because of bugs and clever code in existing software. Otherwise people who upgraded and found their old software not working anymore would have blamed Windows instead of their buggy software.
You can't boast about Firefox being extensible and then blame plugins because they break it. At least, Firefox should be warning about troublesome plugins. The more people have to deal with Firefox being slow and unstable, the more Firefox will be getting a bad rep. Sorry, but you can't fight human nature, you know. You can't change how most users think. If you want to increase market share, your software must be as much dumb-friendly as possible.
On my Linux box, Firefox is crashing lots of times a day without giving me any clues about what could have gone wrong. What am I expected to do? To become a Firefox developer? Switching to another browser is an easier path.
By saying this, I hope I don't seem unappreciative of Firefox developers' efforts.