updated readme to include Firefox/default browser instructions (#157)

README.md

@@ -15,11 +15,19 @@ You might want to check out the [security design doc](/security_design.md).
 
 ![Tor Browser Launcher screenshot](/screenshot.png)
 
+## Using Tor Browser as your default browser, and Firefox
+
+Tor Browser Launcher allows you to set Tor Browser as your default web browser. Unfortunately, there's a gnarly issue that prevents this from working if Firefox is open in the background. If Tor Browser is set as your default browser and Firefox is open in the background, links will get opened in Firefox. Likewise, if Firefox is your default browser and Tor Browser is open in the background, links will get opened in Tor Browser. See more information [here](https://github.com/micahflee/torbrowser-launcher/issues/157).
+
+You can only use Tor Browser as your default browser if you don't use Firefox at the same time. Other browser (such as Iceweasel, Chromium, or Chrome) will work fine. You must check "Allow opening links with Tor Browser" in the settings to enable it.
+
 ## Installing in Ubuntu 14.04 and earlier
 
 I've created a PPA where I'm maintaining torbrowser-launcher binaries. You can install in an Ubuntu-based distribution like this:
 
-    sudo add-apt-repository ppa:micahflee/ppa
-    sudo apt-get update
-    sudo apt-get install torbrowser-launcher
+```sh
+sudo add-apt-repository ppa:micahflee/ppa
+sudo apt-get update
+sudo apt-get install torbrowser-launcher
+```