You have a very old version of Firefox, which can't be updated because the version of Mac you have (10.6) is also very old and no longer supported. This means it's no longer safe to use your computer on the internet.

If your computer is new enough (made in 2009 or later) it can probably be updated to at least a slightly newer version of Mac. This should all be entirely free and will just take a few hours.

You'll have to follow a few steps:

• First, update to Mac El Capitan. Follow the directions at https://support.apple.com/en-us/HT206886 to do so. Your computer can likely support this, as all it requires is a computer bought in 2007 or 2008. Please follow this directions, and install that update ASAP.
• Then, check if your computer supports Mac OS High Sierra. Follow the directions at https://www.apple.com/macos/how-to-upgrade/ to check. If it does, update. You have to update to El Capitan first, then High Sierra, just to make sure you follow the proper update path.

Following those steps will get you on a more modern version of Mac which will be faster, much much safer and secure, and has new features you'll probably love.

AFTER doing that, you can update to Firefox 61. Follow the directions at Update Firefox to the latest release. You need to update Mac first, then Firefox.

After following all these steps (set aside an afternoon to do so, but its entirely free) your computer will be safer, more stable, and faster, and can handle all modern websites.

CMB6523 said

I can use Yahoo mail on Safari, but I want to still use my Yahoo mail on Firefox, where I have all my contacts.

Email Contacts wouldn't be stored in Firefox. They are stored in the Yahoo preferences at their webmail portal.