Firefox (or one of its addons) seems to have problem with some websites. I don't care much, that happens. But as a result the whole system freezes? Mouse still moves but I can't start htop because the desktop doesn't react anymore.
press ctrl+alt+f1 to get a console then kill firefox with pkill command. you can press ctrl+alt+f7 to get back into your desktop. else, as precaution, you can register xkill on a combo button. whenever an app hang you can execute that combo button and click the window to kill that app. if you could launch terminal (usually ctrl+alt+t will bring one) you can also execute xkill from there.
Ctrl+Alt+Del brings up the screen saver, Ctrl+Alt+* should kill the front process but didn't work
not every DE have same combo button. you need to check whether if the combo button is registered/activated. if the combo button is activated, the first combo will log out the system while the second combo does nothing unless you register something under the command. if you want to kill firefox you can register xkill command on that combo button.
Alt+SysRq+something. What key is SysRq? And which of those should I take? They all sound scary.
sysrq is print screen button. the R-E-I-S-U-B combo should be executed one by one with some delay between them. press alt+sysrq+R, wait 3-5 seconds, press alt+sysrq+E, wait 3-5 seconds, press alt+sysrq+I, wait 5-10 seconds, .... normally after the third command your system will restarted in several seconds. if your system is not restarted you can continue with S-U-B command.
since the system is still works (i.e: the cursor is still moving) it means that hard reboot is actually not needed. you actually need to kill firefox either by xkill, pkill, or allkill command. in most cases, only firefox is frozen so you can call terminal either by ctrl+alt+t or using the start menu/shortcut. if you can't access the menu (all the desktop except the cursor is frozen) you can use a console with ctrl+alt+f1 as explained above.