There alredy are many tutorials about the actual installing of linux and how to make it a dual booting system, so this tutorial is not about that. Instead it continues on from there and covers how to setup a common shared partition and have firefox and thunderbird share the same profiles and settings, and email from both platforms.
So you have just installed a flavor of nix you read about, got it dual booted with xp, set up your Internet, email, etc etc. and you start using it. But you still like to use windows as well, you soon realize the pain it is when you find that you have downloaded a bunch of stuff in nix or windows but you can't use or read it from one or the other. You can burn it to cd or copy to thumb drive, and that works for a while. Then you start bookmarking sites you visit, no problem you can export firefox's bookmarks to said cd or thumb drive an re import them to windows but "the pain". Then comes email, well you can set your email client in Linux to not delete email from your provider's server so it is there in windows, but that is a sloppy way of doing things, and i guess there is gmail and the like, but that never really suited me.
Fortunately the folks at mozilla have made these inconveniences an unnecessary experience. However a little fore thought is needed before you setup that dual booting system, or you will have to add another hard drive or edit your existing partitions on an existing dual boot.
First thing to do is to partition your system so that you have a VFAT partition, both windows and linux can read and write to FAT. There are window's drivers that will allow you to read and write to ext2 and ext3 file systems, but writing to ext2 or 3 from windows is not highly recommended, though i have had no issues with it why bother when trusty old FAT is here. So create your partition either when installing nix or create a 32 gig FAT partition from an existing large enough partition that hasn't any valuable data on it yet. then edit your /etc/fstab to include your mount point and permissions. My example is my newly installed PC Linux (nice little distro that installers from a live cd that you can download with nvidia or ati drivers and tonnes of software from apt for rpm). Any way it automatically mounted the FAT partition I keep my mozelle profiles in as C, I think because it is my primary ide drive and i have 2 sata drives the first is actually my C drive in windows, any way it still works and applies for this tutorial, though your system will be different.
You now have a partition with a file system that both Windows and Linux can read and write to.
Now to share the same profile (bookmarks, themes, extensions, etc) you will need now to copy your \documents and settings\user(usualy your name)\application data\mozilla folder to the new FAT partition and perhaps rename it "Mozilla Profiles" or whatever it realy dosen't matter, the screeny above shows mine, and the folder containing my mozilla profiles,you also need to enable hidden files and folders, from the tools, folder options, view menu .
Now back to your origional \documents and settings\user(usualy your name)\application data\mozilla folder and open it, you will see your firefox folder, open it and you will see your profile.ini file. you need to edit it to point to the new location where you have your profile. So open it and change the line to include the new directory path in my case I have tto change the line,
"Path=56fqcw6x.default" to "Path=F:\mozilla Profiles\firefox\56fqcw6x.default" and the line "IsRelative=1" to "IsRelative=0"
Now when I open Firefox when in Windows it will use the profile that is in my F:\mozilla Profiles\firefox\56fqcw6x rather than the default location in my application data folder.
Now boot into Linux and again enable "show hidden files and folders" from your home folder go to view, show hidden files and folders. (this is similar in gnome however I am not as familiar with gnome)
From your home folder open your .mozilla folder, then the firefox folder and edit the profile.ini folder in the same maner as you did in windows, to point to the same profile in your FAT partition. You may find it easier to browse to the location using konqueror and copy the location from the top of the window.
When you run Firefox from Linux you will have the same bookmarks, extensions, themes, etc. however one small point, as the directories for downloading will be different in windows and linux I find it better to simply set downloads so that I am prompted each time as to where I download them to, or you can have firefox in one os to download to the partition and folder of your choice, and the other to the desktop. This is a small price to pay IMO.
Next is email, thunderbird can be setup to use the same email inbox in one easy step. first we will do windows. Simply go back to your FAT partition and create a folder and in my case i call it mail, you can se it in the second last screenshot above. Now run thunderbird and click on the tools menu and select "account settings" then "server settings" near the bottom you will see the path to the folder where email is stored and a browse button you can use to change this to another directory. The screen shots are from linux however it is the same in windows except the menu is in tools rather than edit,
Now copy your \documents and settings\user\application data\thunderbird folder to the folder you have the mozilla profiles in in your FAT partition and edit the profile.ini file in your \documents and settings\user\application data\thunderbird folder to point to it, the same way you edited your firefox profile.ini file and again all your extensions, themes and addressbook will be the same accross both platforms. Also note the directory path is differnt in the thunderbird shots because they are from my SUSE install not the pclinux install.
I hope new dual booters will find this usefull. There are other apps that are on both patforms that can also be edited in this way, however I felt that email and web browsing were the two most important to anyone dual booting.