Locking Firefox with a master password

WinGuard Pro locks any file, folder or program on your computer.

"Well, you can use the UAC feature of Win7 to block it. But then you have to provide password for each application u run.. Anyway, its the best hand you have..

To change the elevation prompt behavior for standard users

1. Click Start, click Accessories, click Run, type secpol.msc in the Open box, and then click OK.
2. From the Local Security Settings console tree, click Local Policies, and then Security Options.
3. Scroll down to and double-click User Account Control: Behavior of the elevation prompt for standard users.
4. From the drop-down menu, select one of the following settings:
* Automatically deny elevation requests (standard users will not be able to run programs requiring elevation, and will not be prompted)

* Prompt for credentials (this setting requires user name and password input before an application or task will run as elevated, and is the default for standard users)

5. Click OK.
6. Close the Local Security Settings window.

Or, another scope is to use a firefox that'll require users other than you to enter a password when they try to access your saved password or bookmarks etc(i.e. your profile data)..
To set that,

Tools > Options > Security tab > Tick ""Use master password"" ..

You'll get a prompt saying what you need to do next.."