Verify you have the correct settings.

https://support.mozilla.org/en-US/kb/thunderbird-and-yahoo#w_sync-your-calendar

Setting are correct. This had been working fine until about 6 weeks ago, now I have a growing list of reminders that will not snooze or dismiss.

Application Basics

Name: Thunderbird
Version: 24.3.0
User Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:24.0) Gecko/20100101 Thunderbird/24.3.0
Profile Folder: Show Folder

(Local drive)
Application Build ID: 20140131124303
Enabled Plugins: about:plugins
Build Configuration: about:buildconfig
Crash Reports: about:crashes
Memory Use: about:memory

Mail and News Accounts
account1:
INCOMING: account1, , (pop3) plus.pop.mail.yahoo.com:995, SSL, passwordCleartext
OUTGOING: smtp.mail.yahoo.com:465, SSL, passwordCleartext, true

account2:
INCOMING: account2, , (none) Local Folders, plain, passwordCleartext

account9:
INCOMING: account9, , (imap) imap.googlemail.com:993, SSL, passwordCleartext
OUTGOING: smtp.googlemail.com:465, SSL, passwordCleartext, true

Extensions
Lightning, 2.6.4, true, {e2fda1a4-762b-4020-b5ad-a41df1933103}
Provider for Google Calendar, 0.25, true, {a62ef8ec-5fdc-40c2-873c-223b8a6925cc}

could you follow the instructions in comment one here https://bugzilla.mozilla.org/show_bug.cgi?id=511098#c1

What appears in the error console?

The attached images indicate the errors and warnings that pop up after clearing the log and then replicating the problem, along with a screen shot of the indicated files and line number.

Just a thought. Is your calendar read-only?
Right-click the calendar in the left pane - Properties

The calendar is not "read-only" I've unchecked "Show Reminders" on this calendar because they keep popping up. On the Yahoo side I've shared the calendar with myself (email id) with view and edit attributes.

Nothing, no fix for this? You all are using the word "support" in support.mozilla.org somewhat loosely.

People answering questions here are volunteers, dedicating their time to help others. Most of them are users like you.

If you don't get anymore replies here that means no one has an answer or solution to your problem.

Have you tried to check with Yahoo about this?

Ok, when the amateur “volunteers” can’t solve the issue what recourse does one have? If a company (even a non-profit) is going to produce a product that is widely used they had better have some sort of professional support organization. If not the product is worthless.

I have contacted Yahoo and since the events that keep popping up in Thunderbird are 5 months old and have been deleted in the yahoo calendar there isn’t much they can do.

You can file a bug in Bugzilla. https://bugzilla.mozilla.org/

But I assume you show as the owner of the event, and are there for allowed to edit and delete it.

This problem came up at the same time on both my Windows 7 version of Lightning and my other computer with the Linux version. That suggests that the problem is something that changed with Yahoo. I can create a new Yahoo calendar and it has the same problems.