Both the Operetta and Operetta Advance themes for Firefox will give you that Opera look and feel you have been looking for.  The main difference between the two themes is that Operetta has the tab bar in the usual location (under the bookmarks) and Operetta Advance will move the tab bar to the top of the browser (on top of the menu).  So you have two choices to work with, depending on where you want the tabs bar.

Here is what Operetta looks like in action.

For comparison’s sake, here is the Operetta Advance theme.

Both themes give a great looking interface, without having to switch to the Opera browser to use it.  You can pick up both the Operetta and Operetta Advance themes (Windows only, for both themes) on the Firefox Add-ons web site.

One new add-on that might change my mind is Integrated GMail, an experimental extension that does a fine job at the previously mentioned task.  It allows you to make the inbox collapsible and loads your choice of Google Calendar, Google Reader, Google Notebook and Google Groups under it (they are also in collapsible boxes).

It was also recently featured on my list of  9 Google Reader Add-ons for Firefox via Mitchelaneous.com.

If your are inbox minimalist, enable minimalist mode to clean up the inbox even more.  This extension is well worth checking out.  You can pick it up on the Firefox Add-ons web site.  Once you do, let me know how it stacks up against other favorite Gmail tools of yours for Firefox.