To be safe and to do something to make it easy for me to check the integrity of the firefox-11.0.tar.bz2, I use sha1sum in Terminal:
Hit Enter and I should see:
Go to this page.
The current link is: http://ftp.mozilla.org/pub/mozilla.org/firefox/
The previous link is dead: http://releases.mozilla.org/pub/mozilla.org/firefox/
Click on releases, 11.0 and SHA1SUMS.
Press Ctrl and F at the same time. Copy the checksum above (that is "96a44eeb3076b4343b55cb21bfd7cd6fc59f5b05") and paste it.
If the checksum is there, this means that the file is intact, and I should probably trust this file.
If not, I should not trust the file.
Seamonkey can be verified in a similar way.
Look for the checksum (SHA-1) here.
The current link is: http://ftp.mozilla.org/pub/mozilla.org/seamonkey/
The previous link is dead: http://releases.mozilla.org/pub/mozilla.org/seamonkey/
This may be useful in Terminal:
What is SHA-1?
For the details, this page may help.