I am using Firefox
I agree with Mr Fattahi
and I want to add this point.

Firefox has is supporting more standards like SVG, XForms, XHTML, ...
so by developing your programs for firefox you can supoort this.

FireFox is great but:
It has problems with helper apps specially with JRE.
If you are running an Applet that uses streaming I/O
chances are that FireFox crashes or even your machine halts.

Firefox is a very nice browser. I like it a lot. But in my opinion, Opera is a
much better browser
with a better overall user experience.

I have both Firefox and Opera installed on my machines and use both of them but
I am more biased
toward Opera.

For a more detailed list of Opera's features have a look at:
http://www.opera.com/products/desktop/ but here comes a small summary:

> > 1. Are you using each new IE page for a single page you visit? ...

Opera supported tabbed browsing a long time before any Mozilla browser. And
Opera's tabbed
browsing is better than Firefox. You can Ctrl+Tab, see a list of open windows,
and choose the one
you like. Version 9 also supports "tab previews" which is even better.

> > 2. Are you tired of seeing Advertisements every where on every ...

Opera also has this nice feature called pop-up blocking however it doesn't work
as good as
Firefox's.

> > 3. How do you download a file from internet? Are you using IE ...

In this regard Firefox is much better than Opera. More than a year  ago I
suggested (in Opera
forums) that it would be nice to add a download accelerator kind of thing to
Opera but it wasn't
welcomed that much. Nevertheless, Opera supports resuming broken downloads as
well but it's not
implemented very carefuly. A plus for Opera 9 is its support for downloading
torrent files (with
full support for resume, etc.) out of the box.

> > 4. Do you want to change the look and feel of the browser? There are ...

Opera's skins are far better than Firefox's IMHO.

> > 1. Do you want to get detail information of each page form element? ...

+1 for Firefox.

> > 2. Do you want to change css on the fly and see what happens? ...

ALAICR, Opera supports this as well.

> > 3. Do you want to know the detail information of you html tables? ...

+1 for Firefox.

> > 4. FireFox versions are more mature. I've experienced that an ...

Opera is just as mature as Firefox (at least.)

> > 5. FireFox can be installed on the Linux and Mac too. ...

Opera too. Plus, it can be installed on your cell phones as well. It's the No. 1
browser for cell
phones both in terms of market share and features.

- You can download Opera from: http://www.opera.com/download
- You can change the skin of your Opera browser  by the click of a single link
(no need to
restart): http://my.opera.com/community/customize/
- You can create your very own blog, photo albums (300 megs of free space), etc.
on
http://my.opera.com for free

What are you going to do with AJAX apps on Opera?

Opera supports AJAX like a charm. Gmail, for example, works fine on Opera.
However, as Opera is
not as widely used as Firefox and IE, many websites are not tested extensively
on Opera. In those
circumstances I usually use Firefox.

Althought fireFox has a lot of advantages rather than
IE and many others, But I should add this notice to
the web  developers who wirte java-script, javaScripts
in IE has a lot of differences with javaScripts in
fireFox ;-) and may be you have to get some
information about it.

I want to add some thing to orods point
actually JavaScript was invention of Netscape (the company and people behind FireFox, FireFox is re engineered version of Mozilla which is open source core of Netscape browser)
Microsoft has JScript and VBScript, and as orod says, JScript and JavaScript are a little different.
the standard scripting language is called EcmaScript which is more similar to JavaScript than JScript

I should add here that FireFox supports WML (Simulator for PDA and Mobiles) by an extension.

This is not such an advantage, WML was never successful and it is going to be replaced (replacement is in progress). lack of support for a standard display content and format, low speed in processing by WAP phones, communication and processing overhead, lack of support by service hosts, also there are so many wap phones in comparison to supporting web  browsers and hosts, etc,

how many people are really using the wap service?

personally I don't see WML support to have a huge added value, these communication and formatting are in the process of replacements,

the only thing I don't like in firefox is that it is not written in java
I read some where that there is a java  version available, does any body know about it?

You can read more

here: http://www.osnews.com/comment.php?news_id=14945
and here: http://my.opera.com/welcome%20to%209/blog/show.dml/306342

You can obtaim Opera 9 for your favorite platform from
http://www.opera.com/download/