No problems here. If you disable the Norton extensions, does that fix it? See Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems

Disabling the Norton tool bar fixed the problem, but that's not an ideal solution. I still think it's something to do with Firefox itself though. After doing more digging, it seems only to happen if the "Open new windows in a new tab instead" option is selected. If I turn that off, it works fine (although it opens in a new window instead of a new tab obviously). And it only happens if I use the "target" attribution of the tag. If I remove the target attribute, it works fine (although again it opens in the same tab instead of a new tab). It doesn't matter how I set the target attribute either - hardcoded in the tag in the HTML, or by updating in Javascript with myanchor.target = "x" or myanchor.setAttribute("target", "x"). It also doesn't matter which value of "x" I use (a special one like "_blank" or a named one like "gregg".

Start Firefox in Diagnose Firefox issues using Troubleshoot Mode to check if one of your add-ons is causing your problem (switch to the DEFAULT theme: Tools > Add-ons > Themes). See Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems and Troubleshoot issues with plugins like Flash or Java to fix common Firefox problems

If it does work in Safe-mode then disable all your extensions and then try to find which is causing it by enabling one at a time until the problem reappears. You can use "Disable all add-ons" on the Safe mode start window. You have to close and restart Firefox after each change via "File > Exit" (on Mac: "Firefox > Quit")

Norton Toolbar is the culprit. I'm working with Norton to get it addressed.