I've been using Seamonkey as the very secure browser to do my online banking, login to my government accounts, in short for all those sites where security is paramount. Perhaps just another Firefox profile would do but I chose Seamonkey because I wanted to get process and profile separation from my usual browsing environment. I wanted a browser without any add-ons so things "just work" on those clunky bank websites.
Enter iMacros. Well, more like enter through the backdoor as I've been introduced to it by Daniel Dawson's Save Password Editor. Daniel has a very useful FAQ page that is worth reading as is. That's where I came across iMacros and I quote him:
For the case of a form created dynamically (as opposed to being written directly in HTML) or employing certain other fancy tricks, the password manager cannot be persuaded to recognize it or cannot fill it properly. In that case, the only solution is to use something else. I usually recommend iMacros for Firefox. Although it's much more powerful than is needed for the task, it's one of the few things that work in almost all cases. You should probably enable its master password feature so that the passwords in your macros will be encrypted.