What are you trying to achieve in the first place?

I have set up rules EG when an email message from a friend comes into my email box it is automatically delivered into a Local Folder marked friends. I can't add any emails to any of my local folders.

So are you saying you created a filter to move certain messages to your Local Folders account, but for some reason the filter does not work?

Yes it applies to all my local folders.

See this article about filters. https://support.mozilla.org/kb/organize-your-messages-using-filters

What are your exact filter settings?

I cannot get to my filter settings. When I try & add an email to the filter it comes up with an error message not responding. the filters still work for existing emails. i just cannot add new email addresses.

Perhaps you might have moved forward some time ago if you had responded to https://support.mozilla.org/en-US/questions/1312377

I will provide the following, which may help you in determining why the file is in use when you try and edit it:

• Restart Thunderbird with add-ons disabled (Thunderbird Safe Mode). On the Help menu, click on "Restart with Add-ons Disabled". If Thunderbird works like normal, there is an Add-on or Theme interfering with normal operations. You will need to re-enable add-ons one at a time until you locate the offender.
• Restart the operating system in safe mode with Networking. This loads only the very basics needed to start your computer while enabling an Internet connection. Click on your operating system for instructions on how to start in safe mode: Windows 10, Windows 8, Windows 7, Windows Vista, Windows XP, OSX

If safe mode for the operating system fixes the issue, there's other software in your computer that's causing problems. Possibilities include but not limited to: AV scanning, virus/malware, background downloads such as program updates.

So are you saying you created a filter to move certain messages to your Local Folders account, but for some reason the filter does not work?

No the filter is working but I can't add new email addresses to the filter.

To Matt Thanks for your ideas: I restarted Thunderbird with add-ons disabled, it still did not work. I restart the operating system in safe mode with Networking & that did not work. I also restarted Thunderbird with add-ons disabled whilst running Thunderbird with add-ons disabled. I am receiving an error message "Warning unresponsive Scripts"

I forgot to mention that I have repaired all my folders.

What are you trying to achieve in the first place?

I am trying to add email addresses to filters I have already set up.

I about to the point of wondering is the file is corrupt on disk. I suggest you run a disk scan using the operating system tools to check for errors.

Instructions here (they look complete and risk free) https://neosmart.net/wiki/chkdsk/#From_My_Computer-5

Thanks Matt I tried that & run the program twice each time with no errors! The filter is working but I can't add new email addresses to the filter. As a temporary work round I have set up a new filter to put new messages into. It was easy to set up & works well. EG The first folder was Finance 1, I have called the second folder Fiance 2. Is there a limit on the number of email addresses you can put in one folder? I have over 200 in the one I can no longer add email addresses to?

I have now created a new filter & added a 2 to it. With this new folder I can direct new email addresses into the new folder. The old folder still accepts emails but I can't add new addresses to it. I have over 200 email addresses in the old folder. Is there a limit on the number of email addresses you can have in any one local folder?

I think the problem is you are trying to add addresses to a filter with a rule such as From + is + <address>, and the filter is not accepting more addresses. Create an address book with the addresses you wish to filter, then change the rule to From + is in my address book + <the new address book>. Contacts can be added to the new book from existing ones by drag and drop, and by clicking (twice) the star next to an address in new messages.

I cannot get into the folder, I am trying to delete some old addresses but cannot access the folder just hangs!