I did because I didn't want to lose my zillions of bookmarks.
You can make a copy of the profile and restore it later, or boot firefox in safe mode to see if that will resolve the issue.
If its caused by a bad extension/plugin this is the most likely spot its sitting.
What is PortableApps please?http://portableapps.com/
It allows you to have a run-only version of Firefox on a USB that contains a self contained profile.
The first time you install it, make sure you don't import any profiles on the computer, otherwise if the issue is profile based within your firefox settings - it will import the problem.
This is another way to test if a clean profile, rather than moving the old folder, try this out first and if you want I can put together some info about the firefox profile.
I have never heard of a Live CD either! Pretty certain I don't have one.
A Live CD is often bundled into many computer magazines, or you can download it from most linux distributions.
If as you said the problem occurs in firefox, most of these would have some form of firefox.
You put the disk into the drive, hit f12, or esc - which ever option your computer supports to bring up its boot menu and boot of the disk.
It wont look familiar but you should be able to get into firefox, and out to the internet.
I can't guarantee it will work, as custom configurations can have hardware that it doesn't recognise.
We have an inhouse tool at work thats a custom WinPE (cut down Win7 Pre-Installation Environment) that lets us do a couple of tests etc, and recover data.
HD SMART test? Sorry, lost me again.
Most manufactures provide a boot disk, or software package that lets you check your HD for errors.
I can't recall the exact meaning of SMART, but its a series of benchmarks that let you know if a drive is close to failure.
If you are getting SMART warnings, you should have time to copy all your data off the drive before failure.
Its a custom built PC. I'm okay with doing simple servicing on it like changing memory but I'd be nervous about pulling out the graphics card and I have no idea if the mainboard has on-board graphics.
My answers aren't much help, but I do very much appreciate all your ideas.
If you have someone that is confident, have them do these checks for you. I reseat graphics cards all the time, but its part of a warranty step in a fleet of devices - rather than a custom built PC.
To check if your machine has a built in graphics, look at the motherboard. What you are looking for is most likely a VGA port - See - https://duckduckgo.com/?q=vga+port
or a DVI port that looks like https://duckduckgo.com/?q=dvi+port
If the port is lies on the same axis as your motherboard - ie horizontal in a flat desktop case, and vertical in a tower case, then its on board. If its on the opposite axis to your motherboard, its on the graphics card.
If it does have these, and your monitor has the same, you can try plugging into one of these, and then going into your BIOS and somewhere in there under video it will talk about using the onboard rather than external video. Sometimes these just work, and allow for multiple monitors, other times they are completely disabled.