If you're a power user (especially a web designer), you might have the need to be logged into a website with more than one account simultaneously. Or maybe you want to increase efficiency by fully customizing one Firefox window for work and another for play and casual browsing. By utilizing multiple instances of Firefox, both are stupidly simple to accomplish - and more!

Instances Are Different From New Windows?

When you run Firefox, that is a single instance. You can open a new window for new tabs and whatnot, but you're still working within that same instance of Firefox. The theme is the same and all favorites, add-ons, history, etc are the same across all windows.

When you open a second instance, it is like having a completely different Firefox. Nothing is shared between instances - not themes, add-ons or even login information. You can customize this instance to be specific to whatever purpose it will serve and it will never affect any other instance.

Why Would This Be Helpful?

Simultaneous Website Logins

Login information for each website is handled separately for each instance. For example, if you have 2 Gmail accounts, you can log in to both of them simultaneously by visiting the site in separate instances of Firefox.

When I design a website in Drupal, it's important to see what the website looks like both when logged in and not logged in. Sure, it's possible to log in, make changes, then log out again, but what a pain! By using two instances of Firefox, you can have one window logged in and another window logged out. Make a change in the logged in window and refresh the page in the logged out window to see the change.

Privacy & Security

It's a pain to view websites with high security settings enabled. While at home or work it may be safe to disable some security features. However, when on public wifi you can never have too much security. To help protect privacy and the security of your computer, it's possible to make a profile which is set up with high security settings add-ons to provide even more security.

Backing Up Profiles

When you create a profile, you can select where the information for the profile should be stored. By changing these locations it's easy to put the profiles in a place that's more likely to be backed up.

Custom Work Environment

This is the biggest use I've found for doing this. I have one profile for work, one for standard misc browsing, and one optimized for speed. Here's what I have in each profile.

Work:

• My work profile is optimized to provide me with the tools I need while making it less convenient to be distracted.
• Add-ons like Web Developer Toolbar & Firebug.
• Only work-specific bookmarks (less temptations from other sites).
• Work-specific websites open:
  • Work Email
  • Unfuddle
  • Basecamp

Default:

• My default profile is kind of slow, but everything I need is at my fingertips. I leave a number of tabs open at all times (currently 10), use lots of add-ons and have a huge catalog of bookmarks.
• Add-ons like Greasemonkey & WiseStamp.
• All bookmarks organized with Xmarks for convenience.

Quick:

• Ideal for doing a quick google search or checking out a website without opening another profile and waiting for the previous tabs and add-ons to load.
• No add-ons or special themes.
• No significant changes from the Firefox install.

Can you think of some other uses? Let me know by leaving a comment at the bottom of this post.

How to Run Multiple Instances of Firefox

As I said in the introduction, it's stupidly simple to set this up. I attached a script in a zip file at the bottom of this blog post. Just unzip that or make your own script by putting the following code in a plain text, TextEdit file with no file extension:

#! /bin/sh

/Applications/Firefox.app/Contents/MacOS/firefox-bin -ProfileManager

If you double click the script, it will launch the Firefox Profile Manager. You'll also see a console window appear, but ignore it for now.

Uncheck the box "Don't ask at startup" to make the profile manager appear every time. You want this to happen so you can select which profile to launch when you start Firefox. Now add another profile for whatever purpose you have for it. This new profile will launch a totally new instance of Firefox from the other profile and will appear just like a fresh new install would.

The Annoying Console Window

Unfortunately, the console window must stay open for as long as the profile manager is open. An easy way around this is to open the script using Quicksilver (you're all using Quicksilver... right?) Quicksilver sort of works like Spotlight, but it does so much more. In this case it will run the script without the need for a console window and automatically detect the script so long as it's in your Documents, Desktop, Applications or a number of other folders.

Multiple Instances of Firefox in Windows

If you're not blessed with a mac and would like to take advantage of this nifty Firefox tip, check out this easy to follow article from LifeHacker or one of the thousands of other articles found by searching Google.

Your Turn

What do you use multiple instances of Firefox for? Are there additional uses that I forgot to mention or an easier way to do it? Let me and everyone else know by posting below.