As you'll notice Firefox cannot be updated in Ubuntu using the about menu bar. The simplest way to upgrade is to remove and install it again. You won't lose your Firefox files like book mark and all.
I have Firefox 26 in my system as in the picture. I'll be updating this. I'll show the updated screenshot in the end.
Steps
1. In terminal (shift+alt+t) enter this code:
sudo-apt get update
This will update the software list to latest version.
2. After this, copy the code below and paste it in terminal:
sudo apt-get --purge --reinstall install firefox
3. After the download and installation is finished, restart.
4. Wolla! You have updated your Firefox Browser to latest version. Here is the proof:
sudo-apt get update
This will update the software list to latest version.
2. After this, copy the code below and paste it in terminal:
sudo apt-get --purge --reinstall install firefox
3. After the download and installation is finished, restart.
4. Wolla! You have updated your Firefox Browser to latest version. Here is the proof: