How do I close or open all disclosure triangles in the Final Cut Pro browser?

For the keyword collections, use my keyboard shortcut method. Press Cmd-A, or use the usual selection commands to select the ones you want. Then use the right and left arrow keys to open and close the triangles.

@BwnB

I just tried it and it only "opened" one clip. It doesn't work.

Oh, you mean press Option and clip the Clips triangle. OK, that works, but sometimes you might only want to open some of them at once. Then you need to select them using the usual methods. I prefer the keyboard shortcuts, esp. with disclosure triangles, for some reason. And Final Cut is the only app I know that has gobs of shortcuts, and they all work (so far, anyway!) You can't say that for most other apps, IINM. Even then, for many things in FCP you either have to use the mouse, or if shortcuts are even available, use a tedious, time-consuming path.