tons of missing emails

[JSwimmer]

About mid day today I noticed that I can only see about 100 of the approx 6,000 emails that should be in my inbox. I have Yahoo email for my Verizon email account and was checking using the Yahoo Android mail app. I checked online via Firefox and found the same thing. I used the Verizon support chat and got nowhere with them. They couldn't get past that I was NOT using Windows 10 and that I was NOT having a problem sending or receiving email. I am receiving email and can send email but am missing 5,000+ emails from my inbox. Everything from before August 19 is gone and not in my trash either. I was hoping they would tell me about an email server hiccup and that some data was temporarily lost, to be restored soon. Apparently not. Anybody have any ideas?

[JSwimmer]

This does not appear to be an issue with Android as when I checked email from home yesterday evening using Firefox/Windows 10 WebMail, still no old email.  This is not a catestrophic loss because my home computer has Thunderbird, which has copies of all of my email.  Thunderbird is set up to download my email bu leave copies on the server.

If accidentally deleted, shouldn't the email, or at least some of them, be in my online Trash?

Which servers, Verizon or Yahoo, is mail in my online inbox stored on?

Finally, is there any way to see if I reached a size limit, i.e. too many emails in my inbox, that might have caused some action to automatically delete some of them?  I would hope that I would receive some sort of message that I had reached a size limit.

Thank you for any comments or suggestions.

[JSwimmer · Accepted solution]

Turns out there's a Thunderbird setting below "Leave messages on the server" that says "For at most 14 days" that I had checked. Oops. So I actually did delete all my mail online older than 14.