Create the startup files and folders via:
https://addons.mozilla.org/en-US/developers/tools/builder/9b0cddaa3fc073e50a0e7e284ef64438

Prefill all the fields, and build the zip file.
Unpack this zip file, and open:
/chrome/content/overlay.js

You can edit this file.
In chrome/skin/ are the files for editing the styles and logo.

After changing the files. Zip it in and rename the zip file to the xpi extension.
Drag the xpi file in your FF browser.. And your own plugin is installed.

More info:
https://addons.mozilla.org/en-US/developers/docs/getting-started
https://developer.mozilla.org/en/Code_snippets/Tabbed_browser
https://developer.mozilla.org/en/XUL/tabbrowser#m-loadURI

Don’t you think it’s frustrating that every time you click on a contact (mailto) link, your Microsoft Outlook will be opened automatically, why you would prefer to write the email with GMail?
Just configure Firefox for this!

Open the inbox of your GMail account.
Write this line in the addressbar:

javascript:window.navigator.registerProtocolHandler("mailto","https://mail.google.com/mail/?extsrc=mailto&url=%s","GMail")

Firefox can ask you to add mail.google.com as an application for your mailto links.\
Now, if you click a mailto: link, try the tips link Firefox 3 will ask which application you want to use.
Choose Gmail, and select “Remember my choice for mailto links” to set the preference permanently.
Go to Tools > Appications > Mailto: set “Use Gmail”

Make sure Firefox is configured correctly in the about:config. (should be by default).

handler.external.mailto = true

Tired of configuring Firefox to it’s best performance?
It’s possible to backup your browser settings and add-on settings and it’s also possible to add new settings just in the preferences file since it’s just a *.js file. That means Javascript.

You can find this file on your harddisk:
XP:

C:\Documents and Settings\<username>\Application Data\Mozilla\Firefox\Profiles\<profile ID>\

Vista:

 \Users\<username>\AppData\Roaming\Mozilla\Firefox\Profiles\<profile ID>.default\

Open the prefs.js file with your favorite dev editor.
From here you can make a backup of this file or add new settings, for example:

user_pref("bidi.support", 0);
user_pref("browser.tabs.tabMaxWidth", 100);
user_pref("browser.tabs.tabMinWidth", 50);
user_pref("layout.word_select.eat_space_to_next_word", false);
user_pref("network.http.max-connections", 96);
user_pref("network.http.pipelining", true);
user_pref("network.http.pipelining.maxrequests", 16);
user_pref("network.http.pipelining.ssl", true);
user_pref("network.http.proxy.pipelining", true);
user_pref("nglayout.initialpaint.delay", 50);

Mozilla Firefox is probably the program I daily use the most. The following tweaks are quiet handy, and that’s the reason why I want to share these settings with you.
To configure Firefox, all you need to do is type in your browsers addressbar:

about:config

(If you enter the config screen for the first time, Firefox will prompt you, if you are sure by making this settings.) From this screen you can filter on properties.
(See my related blog post: How to make Firefox faster)

Number of suggestions
Change the number of suggestions of the location bar dropdown list. Default = 12. I changed this to 5. But to disable it, you can specify -1 as value.

browser.urlbar.maxRichResults = 5

Width of the browser tabs
Change the size of the browser tabs. Default they are quiet huge: 250px width. I changed the size to 100px.

browser.tabs.tabMinWidth = 50
browser.tabs.tabMaxWidth = 100px

Select words on double click
When you double click on a word in Firefox, Firefox selects the word plus an extra
space. This can be annoying since you want to copy the word only not
the extra space. So I set this value to false.

layout.word_select.eat_space_to_next_word  = false

Spell checker on all fields
Enable spell checking in all text fields. The default spell checking function only checks for multiline text boxes (default = 1). You can set this for single line inputboxes as well: 2,
Setting to 0, disables the spell checker

layout.spellcheckDefault = 2

No Javascript popups
When you don’t like javascript popup’s you can let Firefox open them
as new tabs. (default = 2). By setting this to 1, no new windows will be opened. By setting it to 0 the new pop-ups will be opened in a new tab.

browser.link.open_newwindow.restriction = 0

Make it faster
Set bidi.support = 0. You’ll probably never need it, so reduce the number of potential bugs and security issues by disabling it.

bidi.support = 0

Set network.prefetch-next = false to prevent random prefetching of webpages which means wasting CPU cycles and bandwidth, as well as subtle privacy and security issues. http://kb.mozillazine.org/Network.prefetch-next

network.prefetch-next = false

Start rendering pages faster by creating a new integer variable (right-click new) nglayout.initialpaint.delay integer preference lets you control how long Firefox waits before starting to render a page. If this value isn’t set, Firefox defaults to 250 milliseconds, or 0.25 of a second.
Set this to 0 to start rendering immideatly. I used the value 50 which is also quiet fast.

nglayout.initialpaint.delay = 50

As default browser.cache.memory.enable is true.
This setting lets Firefox saving copies of all graphical elements
from the current browsing during a session for faster rendering.
I set this to false to free up more memory. (Note that pages from
your history will reload less quickly when you revisits them.)

browser.cache.memory.enable = true

Firefox caches several recently visited Web pages in memory so they don’t have to be regenerated when you press Back or Forward. The integer setting browser.sessionhistory.max_total_viewers determines how many individual Web pages to store in the back/forward cache; each page takes about 4MB (or 4,000KB) of RAM. The default setting is -1, which stores a maximum of 8 pages. for me 3 is more then enough:

browser.sessionhistory.max_total_viewers = 3

Swap out to disk memory when minimized (Windows only). A little-known feature in Firefox allows the Windows memory manager to swap out some of Firefox’s physical memory space to disk when Firefox is minimized but not closed. This allows other programs to use the physical memory that Firefox was previously monopolizing. Firefox’s minimized memory usage with (top) and without (bottom) config.trim_on_minimize. (Click for larger view.)
By default, this feature is turned off, for two reasons: 1) PC memory is generally more plentiful than it used to be, so it makes sense to use it if it’s available, and 2) swapping Firefox’s memory out to disk will slow the program down when it’s restored.
That said, if you run Firefox side by side with other memory-hungry applications, it might help keep them from competing with each other. To enable this feature, create a new Boolean preference called config.trim_on_minimize and set its value to true.

config.trim_on_minimize  = true

Is this too much work for you, see my related post how you can add them in the Mozilla preferences file, and how you can back-up this one:
http://ladysign-apps.com/blog/archives/215

Here are the browser statistics for October 2008, according W3C:

1. Mozilla Firefox – 44.0%
2. MS Internet Explorer 7 – 26.9%
3. MS Internet Explorer 6 – 20.2%
4. Google Chrome – 3.0%
5. Safari – 2.8%
6. Opera – 2.2%
7. Mozilla – 0.4%