Show hidden files and folders
From MozillaZine Knowledge Base
The operating system hides certain files and folders by default, to reduce clutter and to prevent users who don't know what they're doing from accidentally damaging their system. On Windows, Linux, and Mac OS X 10.7 or above, the profile for your Mozilla application is stored in a hidden location and, to find it, you will need to show hidden files and folders. You may also need to find other hidden files and folders. How you do that depends on your operating system.
Navigating to a folder
You can set Windows to show hidden files and folders by changing your "View" settings in Folder Options, as shown here on Windows XP. You can access Folder Options from the "Tools" menu in Windows Explorer (or "My Computer") or from Windows Control Panel. (Note: Windows Vista disables the File/Edit/View/Tools/Help menu in Windows Explorer. You can temporarily enable the menu bar by pressing the <ALT> key. If you want to permanently enable it click the Organize button, go to Layout and select Menu Bar.)
In Folder Options, click the "View" tab and, under "Hidden files and folders", select "Show hidden files and folders" ("Show hidden files, folders, and drives" on Windows 7). You might also want to uncheck the "Hide extensions for known file types" box, so that you can see the file extensions for all files.
If you still can't see the hidden files, you may need to uncheck the "Hide protected operating system files (Recommended)" box. This should only be needed in rare cases, such as showing certain system files. Make sure you recheck this box after completing your task.
This article includes detailed instructions for viewing hidden files and folders on Windows versions up through Windows 8.
Searching for a file
Windows includes a built-in search tool that can be used to search for files and folders. If you are using Windows 2000, the instructions above will also enable searching hidden files and folders. If you are using Windows XP or Vista you must enable searching for hidden files and folders in the search tool itself.
Linux hides files or folders that begin with a dot. Most file managers (Nautilus , Konqueror, Thunar) have a "View -> Show Hidden Files" command that will make them visible. If it doesn't have that exact command it should have something similar. For example the Dolphin file manager doesn't have that command because it lets you enable showing hidden files by modifying the folders view properties.
Nautilus (the default file manager for Gnome) will not show any hidden files that end with a "~" (such as backup files generated by gedit). You can view them using a terminal window or a different file manager. 
Firefox, Thunderbird and SeaMonkey use the native file picker by default (for example, when adding attachments). However, the Linux builds have a built-in file picker that you can enable by setting the ui.allow_platform_file_picker preference to false using either about:config or the Config editor (Thunderbird). It is typically faster and has a "show hidden files and directories" checkbox at bottom left. Unfortunately that checkbox is not sticky, the next time you run the application you need to enable it again. Here's a snapshot:
You could modify filepickerLoad() in chrome/toolkit.jar/content/global/filepicker.js to call the function to show hidden files and folders but it is not recommended. 
If you enable "show hidden files" in the file manager when Mozilla calls the native file picker it will also show hidden files. You can also right click on the directory listing in the file picker (such as the Attach files(s) window) and select "Show Hidden Files"... if the option exists.
Mac OS X
Showing hidden files and folders on Mac OS X
There are three ways to hide a file under Mac OS X: 
Normally you're only concerned about files or folders that begin with a dot. You can make them visible by typing the following in a terminal window.
defaults write com.apple.finder AppleShowAllFiles TRUE killall Finder
This will also cause any file icons to take on a hazy, 50% alpha look. To restore the old settings (hide the files and make the icons look normal) issue the same commands again, but enter FALSE instead of TRUE. 
If you need to do this often its more convenient to use a context menu (right click). You can do this by creating a shell script, and then wrapping it in an Automator plugin for Finder. Or you could use a Finder substitute such as XFile and PathFinder that have a option to show hidden files and directories. 
You can start the Terminal by:
When you open the Terminal it will show a one line greeting, and a terminal prompt on the second line. The terminal prompt shows the name of your computer plus the current directory. The Terminal provides a command line interface to the UNIX shell, which is just another way to work with your Mac. Rather than clicking on folders, icons or selecting commands from a menu with a mouse you type text commands.
You exit the Terminal like any other application. Click here for a tutorial on how to use the Terminal .
Mac OS X 10.3 and later includes FileVault. If you turn it on it creates a separate volume for your home folder and encrypts the contents of it. Several users have reported that when they updated OSX to 10.5.2 via Apple update that the root file system has a /Users/.username folder for each user. This may hide the profile from your Mozilla application.