Setting default browser manually

You can set the default browser in Windows 2000 (SP3+) Windows XP (SP1+) and Windows Vista using the "Set Program Access and Defaults" feature (renamed "Set Program Access and Computer Defaults" in Windows Vista). [3]

* Windows 2000: "Start -> Control Panel -> Add or Remove Programs -> Set Program Access and Defaults"
* Windows XP: "Start -> Control Panel -> Add or Remove Programs -> Set Program Access and Defaults -> Custom"
* Windows Vista: "Start -> Default Programs -> Set Program Access and Computer Defaults -> Custom" [4]

(Windows XP/Vista: Click the icon to the right of "Custom", to expand the category.)

You will see Internet Explorer and other installed browsers listed under "Choose a default Web browser" . Select "Mozilla" (Suite), "SeaMonkey" or "Mozilla Firefox" (in some cases, "Mozilla Firefox" may not be listed; to add it back, reinstall Firefox [5]).

If the above doesn’t work or if the "Set Program Access and Defaults" feature isn’t available in your Windows version, you can manually set the default browser by selecting it as the the default program for individual file types and protocols as follows:

* Windows XP and earlier: Open the Control Panel from the Windows Start menu.
o In Windows 2000 and earlier, or if Windows XP is using the Control Panel "Classic View": Click on "Folder Options -> File Types".
o In Windows XP, if using the Control Panel "Category View": Click on "Performance and Maintenance". Then, click on "File Types" in the left column under the heading "See Also".
* Windows Vista: Click the Start button, open "Default Programs" and then click "Associate a file type or protocol with a program". [6]

Assign the following protocols and file types to the browser you wish to set as default:

* URL:HyperText Transfer Protocol (HTTP protocol)
* URL:HyperText Transfer Protocol with Privacy (HTTPS protocol)
* URL:File Transfer Protocol {FTP protocol)
* HTML File
* HTM File (optional)

In Windows XP and earlier, you can find the URL protocols listed above in File Types, under extension "N/A" or "(NONE)".

Note: On Windows Vista, you may be unable to associate the HTTP HTTPS or FTP protocols with SeaMonkey because it does not appear as an available option.
References :
Doing it myself and doing a bit of research on the web!

Tools > Options (or preferences)

In the Settings, there should be a way to check if the browser is the default browser.
References :