So in All Programs, you see Mozilla Firefox (not just Mozilla) and it doesn't expand to show further options, including safe mode, see image, click to expand.
If you haven't got a safe mode option, this is what is behind that option in the All Programs link for it "C:\Program Files\Mozilla Firefox\firefox.exe" -safe-mode so you could try running that from the Windows Run window. See image2, you can copy and paste the above bold text into the Run window and click OK.
You shouldn't have normal firefox running when you do this. You will also get a notification that it will run firefox without add-ons and do you want to do this all of the time (or words to that effect), you don't want that.
I haven't noticed any slowdown on my firefox 10.0.1, XP Pro system, nor my win7 netbook when I use that.