Browser hijacking

Recently, I had a bank account hacked and an attempt to defraud me of some money. Following report to bank, I downloaded AVG (already have Malwarebytes) and did complete scan of my system. Nothing was found. Thinking back to what might have happened, I had a recent experience where I was trying to log in to that bank, via Firefox, and after entering all login info and clicking signin, it kicked me back to the beginning of the login page. I tried several times and decided the bank was no longer using Firefox (wrong later found out). Firefox worked with other banks. Could this have been a hijack of my browser? Should I take my computer (Mac) in for further testing?