My online banking requires Firefox 3.5 instead of the default Firefox 4 that comes with Ubuntu 11.04. How can I install version 3.5 to be able to have it running side-by-side to Firefox 4?
Instead of downgrading to an unsupported Firefox version, install the User Agent Switcher add-on and set the User Agent to Firefox 3.5. This makes your bank website believe that you're using Firefox 3.5, but keeps the advantages from using Firefox 4.
It's ridiculous that your bank website is forcing you to use an outdated browser. If you really insist on using Firefox 3.5, follow the instructions in How to install Firefox 3.6 and 4.0 in parallel?, but replace