In my particular use case I wanted links in Gwibber to open in Chromium instead of Firefox. Gwibber is a Twitter client written in Python. Unfortunately all the proposed solutions from above didn’t work. Fortunately I found a solution.
When you are reading a post on Google+ the page might contain the IMHO annoying banner to join Google’s social network. The banner distracts you from reading the article - it even follows you when you are scrolling. This bookmarklet for your browser’s toolbar hides the banner.