If you have the latest Ubuntu (6.10 at the time of writing), the easiest way is probably to install it using apt:
sudo apt-get install firefox
Or if "firefox" installs version 1.5, try "firefox-2" or "firefox2" or something instead. Or just run synaptic
which is a graphical front-end for apt, search for "firefox" and install the appropriate package.
Another way is to download the archive from mozilla's site and extract it: open a terminal and use cd
command to get into the directory where the downloaded archive is. Then unarchive it:
tar -xzvf firefox-126.96.36.199.tar.gz
(change the filename if needed). After this move in to the directory with cd command and try running the executable inside:
(or change the filename if needed).
That works if it works; if you get errors of missing library files, you would need to copy or link the appropriate lib files, which are inside the directory where the executable is, to your system's library directory, but in some cases it "just works" from there. I still prefer the apt/synaptic-way, it makes the process of updating it a lot easier.