In my previous blog, I mentioned installing some extra add-ons. What I forgot to mention was that I had problem initially while trying to install ANY add-ons - an error #228 occurs eveytime. After researching the web, I found that this was a common problem with 64-bit Ubuntu and it had to do something with IPv6 support in Firefox. This has to be disabled before add-ons can be successfully installed.
Following the instruction, I executed 'about:config' in Firefox and then clicked "I'll be careful" button on the warning that appeared. I then typed 'ipv6' in the filter box to get to the one and only line that's needed - that line will say 'network.dns.disableipv6'. The default value is "false". I then double-clicked it to set the value to "true" (to disable IPv6). Once this was done and Firefox restarted, I could install the add-ons without any problems. Note that if you do require IPv6 support in Firefox you need to reverse the process.