You will only see that RSS button on the location bar if a website offers one or more feeds.
You should see such a button on this forum page.

Start Firefox in Diagnose Firefox issues using Troubleshoot Mode to check if one of the add-ons is causing the problem (switch to the DEFAULT theme: Tools > Add-ons > Themes).

• Don't make any changes on the Safe mode start window.

See:

• Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems
• Troubleshoot issues with plugins like Flash or Java to fix common Firefox problems

Thanks cor-el

I was the theme, Glaze Black.

I'd already tried disabling extensions & plugins. never occurred to me that it could be the theme.

cheers