Easy Solution
Ironically, it is probably Firebug which is slowing down Google Maps for you so much. To increase performance, the easiest thing would be to disable Firebug when you're not doing development.
Better Solution
Unfortunately, often times disabling such a useful tool is not an acceptable solution. Thankfully it's only a few components of Firebug that play the biggest role in slowing things down. The biggest performance gains come from disabling the "console" and/or "net" tabs, either by clicking them and deselecting enabled, or going through the options tab. Firebug's performance hit is most noticeable on Google Maps because of the large amount of requests it makes and the complex DOM structure.
Backup Plan
If that doesn't help, you can try updating Firefox and Firebug and hope that they've improved performance in recent versions. The most recent version as of when I am writing this is Firebug 1.10.0a7 for Firefox 12, and on my machine it seems to perform reasonably well even with net and console enabled.
I am using firebug 1.11.1 and cannot seem to find the option to disable firebug for the maps.google.com url and/or gmail.com url. I have found some resources online who say that it will be hidden in a dropdown arrow near the 'net' tab on the firebug panel. but that menu does not show. disable firebug for domain x – JP Hellemons – 2013-01-11T09:17:03.103
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Firebug should by default remember on which URLs you have it enabled and disabled, following the same origin policy. Take a look at http://getfirebug.com/wiki/index.php/Activation for more info.
– AerandiR – 2013-01-13T02:24:44.200