I finally is switching over to Firefox as my main browser. I find that it works better then Opera on some pages. It is also highly customizable. With some customization, I get the best of Opera plus the compatibility of Firefox. For examples:

I can finally open up a new tab (I prefer tab browsing) and have the home page loaded automatically. My homepage is my bookmark page. So that makes it easy to go places.
I now have the easy option of openning up a new window or a new tab.
I can use the middle button to open a link in a new tab.

Don't forget to installs some extensions to get all the extra features. Currently I am using:

ForecastFox -- international weather
status bar Clock -- show date/time in browser, so that I don't have to drop my mouse down to the auto-hidden windows start bar to see the clock, and mouse over to see the date. Note: There seems to be a problem with the installation source where the file is incorrectly marked for version 0.10 instead of 1.0+. You can work around it by changing the app version or find a repackaged version on the net.
session saver
Add bookmark here - Now I get one of the nicest UI feature of Opera working in Firefox -- This feature let you navigate, using the bookmark drop down menu, to the place where you want to add the bookmark, and a "add bookmark here" selection is available right there. This is much more intuitive. Try it. You'll see what I mean.
The Web developer extension let you examine a HTML page, look at the forms, CSS, tables, and about one thousand other things. A must have if you are developing web application and need help (who doesn't) to debug your generated HTML forms and laouts.
Bookmark Synchronizer to share bookmark between installation. See my short documentation on how to set it up.

MeasureIt let you measure pixels on screen, useful (at least for me) in spacing and designing layout.
Tabbrowser preferences, updated to work with firefox 1.0.

Nowever, there is a known problem with interaction between Eudora and Firefox. If you have Firefox as the default browser, clicking a link in Eudora will open up two windows in Firefox. There is a known fix, which involve deleting some registry entries. Read about it here

BTW, this Gripes about Firefox page has lots of useful information.