When browser freeze happened to me it took me weeks to remember to have System Monitor already open and some unidentified weirdness woud not stop spawning and eating space. killed the process. forget which one. maybe deleted stuff too.
when system froze with mouse and keys and clock and internet a while back once upon a time, that turned into 100% hardware issue (edit: hdd)
note if the clock freezes too before next hard boot. If so then its really a system freeze and not a browser freeze?