Reset Firefox?

[anaxcrosswords]

After weeks of fiddling, Firefox is still pitifully slow. It’s reached the stage where – as an example – YouTube videos play for about 10 seconds then stop, not even attempting to buffer (same happens if I reload the page).
I’ve got the option of resetting, the text above the reset button being:

If you’re having major problems which you can’t resolve, start fresh with only your essential information.

Have to say I’m tempted, but I can find no information about what’s removed to leave only ‘essential information’. Anyone know?

I’ve mentioned elsewhere that I’m likely to have fibre optic broadband installed to correct my current speed of (usually) less than 1mb. What’s making me cautious is that my netbook, wirelessly connected to the same router, has no problem at all online; YouTube videos, for example, run perfectly clearly and I can’t remember it ever pausing to buffer information.

[Buenchico]

That sounds more like a problem with Adobe Flash Player (which is what is actually used, within Firefox, to view Youtube videos) than with Firefox itself.

To find out if it is, go to your list of installed programs (via the Control Panel and 'Add/Remove Programs', 'Programs & Features', or whatever your version of Windows calls it). Uninstall 'Adobe Flash Player Plug-in'. Restart your computer. (That's important!). Then download and reinstall Flash Player from here:
http://get.adobe.com/flashplayer/
(NB: Deselect the 'Optional Offer'!).
Then restart your computer again. (That's nowhere as important as the first restart but I'd still advise it). With a bit of luck, Youtube might then be OK.

If you do decide to reset Firefox, go to Tools > Extensions > Add-ons first, and note down the names of any extensions that you want to keep, as you'll need to reinstall them after resetting. However most other things (such as your bookmarks) are unaffected by resetting. Details here:
https://support.mozilla.org/en-US/kb/reset-firefox-easily-fix-most-problems