As of FoxyCart v2.0, we officially support (with our default templates):
Firefox, Chrome, Safari, Opera on desktop and mobile, for all versions within the past ~3 years.
IE10+ with full functionality.
IE9 without the default Sidecart cart, but with full functionality.
IE8 without the default Sidecart cart, with reduced functionality. See note below for more info.
Accessibility and screen-reading add-ons. We're endeavoring to support assistive plugins and technologies as much as possible.
Mobile and Tablet browsers for modern devices (ie. Android, iOS, but we don't test against super old or feature-limited/non-graphical mobile browsers like Opera Mini).
If you have questions or problems with specific browsers, please let us know on our forum. Note that we likely can't assist too much with browser-specific styling issues on your own custom templates, but we'll do what we can.
IE8 isn't a pleasant browser to work with, and thankfully sees under 4% of global usage (and falling) according to most metrics. That said, we still are attempting to support it. As of 2014-11-26, the state of IE8 support is as follows:
Basic functionality works. This includes coupons, discounts, taxes, and shipping
Checking out as a guest works.
Checking out as a new customer and entering a password to create a new account works.
Our default templates actually look pretty good, but since IE8 doesn't support the placeholder attribute for inputs, those are always visible and not the prettiest.
Checking out as a returning customer doesn't work.
The postal code → city+state autocompletion on the cart doesn't work. This is an annoyance, but it doesn't error or present major problems beyond not presenting shipping and tax estimates from the cart. (This functionality works fine on the checkout.)
Certain custom checkout fields on the checkout might cause problems. In particular, if you have a required checkbox on the checkout and the customer attempts to checkout without checking it, the checkbox might get stuck in an error state.
As with all things, if you're building a site where IE8 support is required, we recommend doing your own testing. Some of the above issues we'll be working on, but others we are unlikely to fix, and there may be other specific edge cases that we haven't yet caught.