Upate on Win10 versus Win7 and UEFI versus MBR

Now that Windows 10 has been out for a while I'd like some opinions on where it sits versus Windows 7.  I have continued to tell my Windows 7 customers to stay there.  These are mostly Windows at home people who don't have confidence using or upgrading computers.  I tell them that if it works don't fix it.  Whenever you upgrade there's a chance of disaster.  

For any Windows 8 customer I say to upgrade.  For both Windows 8 and 10 customers I put in Classic Shell to make it look like Windows 7.  Microsoft keeps on fixing things that aren't broke, disastrously.  

I just bought a new Optiplex 7020 running Windows 8 Professional on it. In terms of performance or for any other reasons should I put on Win7 Pro or go up to Win10?  I'm also wondering if I should go with UEFI or MBR.  What is the advantage of UEFI.  So far I've just found it to be a pain in the behind.  Including, last week, a terrible problem with UEFI on a Lenovo Desktop that nearly bricked the computer.  Soured me on UEFI.

Finally I have a nit question that has nothing to do with hardware. Somehow my Firefox settings have changed so in the HTML options toolbar above I can't see the B for Bold, I for italics, etc.  The toolbar is there but there's nothing in it.  I can see them when I use Opera or Chrome but not Firefox. Does anyone knows what the heck I changed and how I can change it back?

Thanks,
Al