Last minor update: 1 May 2004.
Last major update: 8 October 2003. Tested version: 1.6; though you'll find many older references to 1.4

Mozilla is the biggest hope and the biggest fear of web developers throughout the world. Hope because it offers a chance (though not a good one) to break the Microsoft hegemony, fear because it took three years of development to produce a good browser. The Mozilla project has produced a code engine which will form the heart of new browsers and continues to improve it.

For historical Gecko/Mozilla/Netscape/whatever notes, see the separate Mozilla History page.


If your DHTML doesn't work, it's because your scripts don't yet use the W3C DOM, even though the necessary code was posted on this site in August 1999. Please don't blame the browser for it.

Names and versions

Unfortunately it's impossible at the moment to actually give the names of this code engine and these browsers.

Confused by all the names like Gecko, Mozilla 1.0 (or 5.0?), Netscape 5 (or 6 (or maybe 7?)?)? Read my article about the many names of this browser.

There are far too many Mozilla versions, too. I don't test every single one of them, but instead upgrade about twice a year. Currently this site is switching from 1.4 to 1.6.

In addition the Project has branched Firebirdfox off the regular Mozilla's and does its damndest to change its name every few months or so. Firebirdfox is a browser-only application, while Mozilla also contains a mail program and a news reader. I do not test Firebirdfox, because there is no 1.0 release available yet.

Fortunately the Mozilla version powering a Firebirdfox version can be found in the user agent string (rv: 1.5, for instance).

Please do not ask me to test anything in a newer Mozilla. It's one of the most irritating questions I regularly get, and personally I consider people who ask (or demand) this lazy. They could easily do the required tests themselves, post the results somewhere and send me a link. But no, they're only interested in making stupid remarks.

"Improved performance"

If you check the "Improved Performance" box when installing Mozilla, or later in the Preferences, you could get a nasty surprise. All Mozilla's on your system will start up as the latest one!

I have many Mozilla versions next to each other, and that worked fine, though sometimes you could only run one at a time. No problem. When I decided to improve performance, though, nasty things started to happen.

I downloaded Mozilla 1.6 and installed it next to 1.3 and 1.4. The first time I needed 1.4, though, double-clicking the mozilla.exe in the correct directory started Mozilla 1.6! Apparently the "Keep in memory" function starts up all mozilla.exe files as the version that's kept in memory.

Then I downloaded Netscape 1.7, whose code engine is Mozilla 1.4. When I'd installed it I did the test I needed 1.4 for, and then switched back to Mozilla 1.6 to resume normal work.

All my preferences had been destroyed, including my bookmarks! Fortunately the bookmarks were moved to Netscape 7.1, so I could retrieve them, but that's not the point.

The point is that Mozilla should keep its bloody hands off the preferences of previous installs, or the installs themselves. Copying preferences is of course a service to the end user, but moving them isn't, not in my book.

Bug list


Mozilla has published a copy of the CSS1 spec annotated with Mozilla bugs.

One very odd bug is that Mozilla sometimes completely ignores a style sheet. The <link> tag is correct and leads to the correct style sheet, but Mozilla just refuses to show it.

In older Mozilla's the cause may be that the file's MIME type is text/plain instead of text/css. Note that this does not have anything to do with the type in the <link> tag, it's a server setting. Make sure that your web server sends the text/css MIME type with all style sheets.

More modern Mozilla's, though, do accept a text/plain style sheet, although they print a warning in the error console. If resetting the MIME type doesn't help, it's a case of encoding. Your page uses Unicode, and switching it to ANSI makes the styles visible in Mozilla. Again, this has nothing to do with any character definition in the page itself, but with the way the server encodes the pages.

I do not understand encoding at all, so I cannot explain the difference between Unicode and ANSI but I do know that when you open a page that doesn't show its styles in Textpad, and then do Save As and select ANSI from the lowest select box, the page works fine.


Unfortunately Mozilla supports "doctype switching". I don't closely follow developments in this newest scourge of web design so I cannot offer any more information. Advice: don't use it.


I made these notes for an earlier Mozilla version. I don't know if they're still valid.

It sometimes has trouble with an XHTML 1.0 Transitional DOCTYPE. If you use XHTML Trans and lots of tables, the tables may develop strange and inexplicable bugs if you use anything else than this DOCTYPE declaration:

<!DOCTYPE HTML PUBliC "-//W3C//DTD HTML 4.01 Transitional//EN"

Apart from the official Mozilla Web Authoring FAQ and the Doctype Switching page I found a very helpful page about doctypes in Netscape 6 and Explorer 5 on Mac. An acquaintance of mine also wrote an article about this problem. The page it's on has a wrong DOCTYPE, so take a look at this in Netscape 6 to see what you're up against. See also Richard Bennett's doctype page.
(Some readers suggest that this table behaviour may actually be standards compatible. This is possible, I don't know. In any case a web developer must be prepared for such things).