• "Clear the Cache": Tools > Options > Advanced > Network > Offline Storage (Cache): "Clear Now"
• "Remove the Cookies" from sites that cause problems: Tools > Options > Privacy > Cookies: "Show Cookies"

Start Firefox in Diagnose Firefox issues using Troubleshoot Mode to check if one of your add-ons is causing your problem (switch to the DEFAULT theme: Tools > Add-ons > Themes).

See Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems and Troubleshoot issues with plugins like Flash or Java to fix common Firefox problems

Fifth-Third did a major upgrade this past weekend, if you are seeing any differences in their services now in comparison to last week contact them.

I can't comment about your particular issue, because I haven't used that feature to view canceled check images in the last few years.

The change that I noticed was that the Username / Password on the Login page now has autocomplete=off which is a good change, IMO. Also, I had to change my online password (some were lost during the transition they told me). I was able to get Firefox to save the new password (using the popular bookmarklet), but it won't autofill properly not matter what I try - at least the User ID does.