We can download it
here (for windows users) and
here (for other operating system). And for older version users we can automatically or manually update it.
Automatic updates:
By default, Firefox is configured to automatically check for updates for itself.
3.5:
1. When an update is available, the Firefox Update dialog is automatically displayed. It contains a list of updates for Firefox components that are installed on your system. To begin updating your software, click OK. The new versions of any selected updates are downloaded and installed.
2. When the install process is complete, you must restart Firefox. To close Firefox and re-open it, click Done.
3. After Firefox restarts, all updated versions of Firefox components are available.
3.0:
1. When an update is available, the Software Update dialog is automatically displayed. It contains a list of updates for Firefox components that are installed on your system. To begin updating your software, click OK. The new versions of any selected updates are downloaded and installed.
2. When the install process is complete, you must restart Firefox. To close Firefox and re-open it, click Done.
3. After Firefox restarts, all updated versions of Firefox components are available.
Manually check for updates:
1. To manually check for a Firefox update, click the Help menu at the top of the Firefox window on the menu bar, and select Check for Updates....
* If it is grayed out, see Check for Updates is disabled.
2. If updates are available, click Get the new version » to begin the process.
*If no updates are available, click Finish to close the window.
3. When the install process is complete, you must restart Firefox. To close Firefox and re-open it, click Restart Firefox Now.
4. After Firefox restarts, all updated versions of Firefox components are available.
5. Any add-ons you have installed, which are not yet compatible with the new version will be listed. You can choose to check for updates to those add-ons or check later.
Don't waste your time, update yours
source: http://www.mozilla.com