Woke up yesterday to a broken browser; System Handler

woke up yesterday and suddenly my browser is largely non-functional.

When opening a file browsing window to upload or attach a file, where firefox would normally open my OS's file browser (Dolphin) it now opens a hideous firefox tab with extremely limited functionality (no file sort/ search/ open, or access to most of my system), and the Download toolbar is now completely broken. Clicking a file opens a little dialogue box that asks "Open with System Handler?" and does nothing upon clicking OK. Nothing also happens on clicking the folder icon to the right, which would normally open a Dolphin window with the file in question selected.