Sync is built into Firefox 4+ versions, no need for an add-on like with Firefox 3.5 & 3.6 versions.

Hit the orange Firefox button and select Set Up Sync... to get started on Firefox 5.0.1.

Thanks--I got to "set up sync", however when I go to "Add a device" the computer says to enter the passcode shown on the screen into my iPhone. the phone already shows a passcode and there is no way to change or correct it. What do I do now?

You have me confused now. I thought you said that "sync" was set up on the iPhone already. You posted here with Firefox 5.0.1 on a Windows Vista PC, so that's what I assumed you were referring to when your mentioned the Firefox Sync add-on. The iPhone and iPad use an app named FirefoxHome which is the "sync" app for those devices.

What device are you trying to set up now?

I have Firefox Home installed on my iPhone 4. With your help, I installed the Firefox Sync add-on to my computer--Windows 7 PC . Now my computer wants me to go to my phone and enter the passcode in Firefox Home shown on the computer screen. My phone has a passcode already shown which is unchangeable and is not the same code as the computer (that code changes every 30 sec. or so). My phone will not progress past the already entered passcode, so I am "stuck" and can't get anything set up on my phone beyone that same passcode. It does give me the option to manually sign in and then enter the lengthy Sync Key. Then I would have to answer whether to use "Custom Server" or not, and if so, enter the Server URL. Any help with this passcode issue? And should I enter the info. manually as mentioned above? Thanks! After my phone, hopefully, gets set up, I would like to set up my iPad 2 also. Sara

How to set up Firefox Home on your iOS device (iPhone and iPad)

Take a look to the article. You are doing it in the wrong order.