Shoutbox

Firstly - while I was browsing, I found the roadmap to Firefox 2.0:
http://www.mozilla.org/projects/firefox/roadmap.html

Secondly, you can look at the roadmap to Firefox 1.0. It can be viewed HERE. It clearly states the first release was in 2002, so about 2 years to fully develop a web browser :

0.1     Pescadero     2002-09-23     Bugs     Initial release introducing customizable toolbars, simple new interface, performance and size improvements, and lots of destruction.

0.2     Santa Cruz     2002-10-01     Bugs     Improvements to toolbar customization, inline web form autocomplete, sidebar, extensions management, making preferences work (somewhat), more size improvements and bug fixing.

0.3     Lucia     2002-10-14     Bugs     A few more features, global Go menu, improvements to pop-up blocking, tabbed browsing improvements, redesign of bookmark groups, more speed and size improvements and lots of bug fixing.

0.4     Oceano     2002-10-21     Bugs     Shifting from features to bug fixing and polish. Plans to leverage the Mozilla 1.2 stability push to make Phoenix rock solid.

0.5     Naples     2002-12-07     Bugs     Polish and cleanup. Lots of bug fixing.

0.6     Glendale     2003-05-16     Bugs     Redesigned Options window, new default theme, smooth scrolling, automatic image resizing, and much more.

0.6.1     Glendale     2003-07-28     Bugs     Security and other critical fixes, middle mouse button pan scrolling, a few other fixes picked up off the trunk.

0.7     Indio     2003-10-14     Bugs     Updated helper applications management, automatic downloading, plugin control, bookmark panels, theme updates and numerous bug fixes.

0.7.1     Three Kings     2003-10-25     Bugs     Maintenance update for Mac OS X - focusing on a small number of critical bugs that were present in 0.7 (see link) including external URL handling (when used as default browser) and page Save-As.

0.8     Royal Oak     2004-02-09     Bugs + - ?     Download System Upgrade (ongoing Helper Applications and Download Manager updates), Part II (ben), XPInstall UI Upgrade (ben), Windows Installer (ben), Add Bookmark Dialog UI (pch)

0.9     One Tree Hill     2004-06-15     Bugs + -
?     "Feature Complete" - Linux Installer (bryner), Mail Integration UI (mail button option for toolbar to launch default mailer) (ben), Extension API Changes [chrome registry changes, extension update UI] (ben+et.al.) SmartUpdate (ben) Migration Engine (ben), Help System Documentation Integration (rlk)

1.0PR     Greenlane     2004-09-14     Bugs + - ?     Bugs with higher complexity/risk, localization impact. See queries.
Release Candidates: RC1 (Mission Bay - 2004-10-27) and RC2 (Whangamata - 2004-11-03)

1.0     Phoenix     2004-11-9     Bugs + - ?     Ultra Critical Bugs, PR Feedback

1.1     Deer Park     March 2005     Bugs     Re-sync with trunk. Ongoing HIG compliance work for Aqua, GNOME etc.

1.5      "The Ocho"      ??? 2005      Bugs      First major half of 2.0 development.

2.0         ??? 2005     Bugs     Second major half of 2.0 development.