I have already googled the question and got an incredible answer but I have a problem with replicating it. My windows directory is on the partition called 'OS' and my username is Siba while the firefox profile is 'r0hktjlz.default'. Basically it goes like this:
OS/Users/Siba/AppData/Roaming/Mozilla/Firefox/Profiles/r0hktjlz.default
How should I rewrite the Terminal text that is in the answer?
The answer is from PleaseStand:
Assuming that your Windows partition is mounted as /mnt and that your Windows username is hey, first copy the (randomly named) old profile directory to your Ubuntu account:
cp -r /mnt/Users/hey/AppData/Roaming/Mozilla/Firefox/*.default ~/.mozilla/firefox/win7profile
Then quit Firefox and edit the ~/.mozilla/firefox/profiles.ini file, changing StartWithLastProfile=1 to StartWithLastProfile=0 and adding the following to the end:
[Profile1] Name=win7profile IsRelative=1 Path=win7profile
Restart Firefox. It should then allow you to select which profile you want to use.