You can have Firefox check right now if it is the default browser:
Tools > Options > Advanced > System Defaults
Then click Check Now.
You will see a prompt which will tell you if it is or isn't. If it isn't, simply click Yes and it will set itself as the go to browser for your computer.
On top of that, you may also need to check the following box:
☑ Always check to see if Firefox is the default browser on startup
When you re-installed YIM, it might have reset your default browser to Internet Explorer.
If that still doesn't work, Firefox might be only thinking that it's set as the default. In which case, go to the Control panel for Programs:
Start > Control Panel > Add or Remove Programs > Set Program Access and Defaults
Then open up the Custom configuration and set Mozilla Firefox as your default browser.
Click OK and YIM should now open up any links in the default browser, which has been set in this case as Firefox.