I'm running Firefox 18.0.1 under Windows 7, all updates current, on a Dell M6700 I7, 2.77Ghz, 8Gig ram. When I first start Firefox, it runs fine. After about 30min, it gets slow. the bookmark memu takes several seconds to draw, sites take several seconds to load/reload. This was not a problem with the same version under Windows XP using even more plugins and extensions.
If I quit and restart Firefox, it runs fast for awhile, then slows down again.
I've tried turning off most extensions and plugins and the problem does not change.
Which security software (firewall, anti-virus) do you have?
A possible cause is security software (firewall,anti-virus) that blocks or restricts Firefox or the plugin-container process without informing you, possibly after detecting changes (update) to the Firefox program.
Remove all rules for Firefox and the plugin-container from the permissions list in the firewall and let your firewall ask again for permission to get full unrestricted access to internet for Firefox and the plugin-container process and the updater process.
Create a new profile as a test to check if your current profile is causing the problems.
See "Creating a profile":
If the new profile works then you can transfer some files from an existing profile to the new profile, but be careful not to copy corrupted files.
I tired shutting off all security software and disabled the firewall with no effect. I didn't really expect any change since quiting and restarting Firefox temporarily solves the problem. This has the feel of a memory leak.
Yes, I created a new user acct to make sure it was clean. Watching memory with the Resource Monitor is showing memory for all four categories going up over time. At launch they are 242812, 228056, 52120, 175928 After about 30min of browsing: 580996, 598384, 84984, 513400
Does it help if you run Firefox in Safe mode?
In Firefox Safe mode these changes are effective:
Try to disable hardware acceleration in Firefox.
A lot better with the hardware accelaration disabled! For the record my laptop uses Nvidia Optimus with a Nvidia NVS 4200m (Driver 126.96.36.1997) for graphic intensive apps and an Intel HD (188.8.131.528) for 2D apps.