My safari immediately freezes upon opening. Has been doing this for a day now. Totally, utterly useless.

Quit the Safari app if it is open. To do that click and hold the Safari icon in the Dock and choose Quit from the menu that appears.

• If the application is not responding, "Application Not Responding" will appear "greyed out" at the top of that menu. Select Force Quit from the bottom of that menu.
• Or, How to force an app to quit on your Mac - Apple Support

Then, press and hold a Shift key while launching Safari again. That action opens Safari in windowed mode (not full screen) without opening its previously opened windows or tabs.

Release the Shift key when the Safari window opens.

I've been having the same problem, and I can't even get this far ^ When I click extensions, the rainbow of death starts swirling and I have to force quit. Safari suddenly stopped working yesterday and I have tried all the usual tricks to try to get it to work, to no avail. Currently typing this on chrome, which is working.

If you are able to get Safari open doing what John Galt said, go to Safari > Settings > Extensions. Uncheck any extensions, if any, that you have there. Once you know Safari is stable again and you have extensions that you want, add them back one at a time to make sure they are playing nice.

And booting into Safe Mode has the same problem or are you able to check your Extensions then?

Tried this, tried safe mode, tried reinstalling OS. Now app wont even open.

Well a Safari update just appeared and it fixed the problem as soon as I installed it!