Also happening in Safe Mode. Uninstalled all plugins and add-ons. Refreshed Firefox - so now it only has the 2 default addons (OpenH264 and Adobe Primetime CDM) Left it with default settings so it only opens the Firefox Start page (tried previously only google.ca homepage, tried a local homepage from C:\... on my computer ). Whether I am using Firefox or just let it sit open, it becomes unresponsive in under a minute. For the past several weeks, in version 42, it would only become unresponsive after several hours of use, multiple windows open. Memory (Private Working Set) usage by Firefox would climb up to 400-500K and my whole system would block up until I ended the Firefox Process. Even with a single window with one tab in use, it would rapidly climb to 100K immediately. It still does that with 43.01. I am not seeing usual 200-300K+ I'd been seeing because 43.01 is just not staying open long enough! Once the process ended, the rest of my system resumed normal operations. These last couple of days with 43.01 it's impossible to use at all. I think I've managed to send off 2 crash reports today, but out of a dozen episodes, even the crash reporter is not managing to appear. Leave it for 20+minutes, it won't resume by itself, needs to be killled in task manager. Right now it's at 115,00K and stopped at unresponsive with Mozilla start page and about:addons open.