Auto spacing in Firefox search bar does not work
This is a known bug.  It seems like it was introduced with 4.2.2.  Downgrading to before that is not a good option, but might be available to you.

It affects multiple browsers with multiple keyboards, so I assume the issue must be with the specific type of text field used for their address bars.  I used to encounter this with Chrome but haven't lately, so you could also try that browser instead of Firefox.  In the thread above, someone also mentioned that the stock Google keyboard did not cause the issue for them.