Not able to get new email on Thunderbird

My wife has been using Thunderbird successfully for many years (and we also donate to Mozilla from time to time) - she's been using her email account ([edited] @frontier.com) for at least 15 years, even after it was transferred over to the Yahoo email server a couple of years ago. Also, she has Windows 11. Just in the last week, she has experienced serious problems (messages duplicating themselves over & over, & then not being to get any more new email at all (last email she got was on June 28th). We've upgraded to the latest version of Thunderbird, & I even completely uninstalled & re-installed Thunderbird. I'm at the end of my knowledge & tried everything I know. I'd even be willing to pay for someone's services if we can get these problems fixed. Thanks in advance - my name is Peter Ryall (cell #[edited]) and my email is [edited] @gmail.com, since she's not able to get new email right now. /edited emails and phone# from public community support forum.