Narrow Editor and Broken Link Button?

I’m having problems with the editor.

1) The editor is too narrow – I can only type in about 1/3 the width of the window. There is no resize handle for the box. Clicking the Visual or Text buttons doesn’t seem to do anything.

2) When I select text and click the “link” button in the toolbar, it seems to pop up at the far bottom left of my screen, under the left-hand menu. Since it’s off the screen I can’t use it. I have to hit escape to get out of the dialog and add the link code manually.

Under “Screen Options” is a checkbox for something about distraction-free editing; I unchecked that which meant I can now see the title field for my posts. I don’t see any other settings anywhere.

This is in Firefox 39.0.3 on a Mac. (Just updated to 40 – still having trouble.)

Has anyone else had this problem? I’ve searched but the only references I’ve found are to plugin issues, and the only plugins I have active are Akismet and Jetpack.

Thanks for any replies.