I manually installed Firefox Developer Edition, added it to the dock, and set it as my default browser. All was good, but now it displays a secondary gear icon in the dock when I open it by clicking a link elsewhere in the OS:
Edit with two details to add:
- No matter which way Firefox Dev launches, it keeps its state (remembering open tabs, etc.). So if it launches via an external click of a URL, then I quit and reopen using the dock icon, it still has that tab open.
- If I already have Firefox Dev open via the dock or Applications launcher and then I click a link in another program, it opens a new tab in the open Firefox Dev instance and does not add that other icon to the dock.