I have a Samsung S4 mini, stock Jellybean ROM. I have several browsers installed, one of which is Firefox configured for browsing through a proxy. When I tap on an NFC tag, Android opens up Firefox - I want it to open up a different browser. How?
Normally, only apps that specifically support NFC will work, not just any browser. If you've tried resetting Firefox as a default app, as Izzy suggests, and you still don't get a choice of browser, it may be because your other browsers don't support NFC.
In this case, you can use my app bURLy to choose any browser. It's a free download from Google Play, and it supports NFC and lets you choose any app that understands the URL in the tag: whether it's a browser, the YouTube app, the Facebook app, or any other site's app, regardless of whether the app supports NFC.
You can choose the app each time if you like, you can set a single default app, or if you like, you can add rules for particular URLs, sites, or other conditions. For example, you might want one site to open in Firefox, another site in Chrome.