So...suddenly my Firefox (v. 38.0.1) is really laggy, with slow-to-respond buttons, scrolling etc. So I manually disable all addons, but to no avail. Then I try safe mode and it runs fine. Great. So I enable my addons again one by one, until they're all enabled and it's still running fine. My conclusion: it can't be the addons. What could it be that's causing this? I have updated all my addons and only have Silverlight, Flash, Google updater, Shockwave and a couple of others. One solution would be for this mode - Safe with addons running - to be my default startup mode, but I can't work out how to do that. It's no huge problem because I can make Firefox run fine, but it's a bit time consuming. Any ideas?