They are disabled for a reason: upgrades break things. It would be horrible if they had to do like Microsoft and keep backwards compatibility for 1000 years.
I usually don't do that because extensions break. And they're called extensions, not plugins. Plugins are entirely different. Flash is a plugin.
Memory is usually not a problem, but the lagging and cpu hogging is a problem.Memory leaks? Resource hogging? I can sympathize if we are talking about a low resource computer. Otherwise don't put them in the same sentence as skins and plugins. If you are so resource conscious, don't use them. Not only they use your oh-so-precious memory, but most memory leaks in FF can be traced right to them.
At most, FF has used around 600 MB before I restarted it. If that was the only problem, I could leave it running until it used 2 GB, unless I was hurting for memory (it's amazing how easy it is to get to 4 GB commit).
And, believe it or not, I have tried disabling all extensions and plugins and the issue still persists. Leaks. CPU hogging.