For every filter I get a message saying that the filter could not be applied.

Please post a screen shot of this error and exact text.

If you have trouble making a screenshot, this may help: https://support.mozilla.org/en-US/kb/how-to-make-screenshots or reply for more assistance. thank you

re :I also had the error, it randomly started moving messages to folders.

I presume you mean you had filters moving emails into folders and they should not be in those folders.

Check your filters. If any condition in any filter has a blank string: example: 'Subject' 'contains' and then the text area is left blank

Then you need to remove that blank string - delete the filter and recreate it with no blank strings.

re :now I'm not getting new messages. For every filter I get a message saying that the filter could not be applied.

Filters automatically get applied to new incoming mail in the Inbox. If no new incoming messages, then filter cannot be applied. Sounds logical. That's a general guess because maybe you are creating filters that have blank strings or illogical or maybe there is no connection to network, so moving an email is not able to done. It's important to offer information and that means you need to upload requested image files that means they have eg: jpg extension

However, you are also talking about 'now I'm not getting new messages. ' Stop/disable all your filters so they are not involved. click on 'Get Messages' What happens ? Do you get an error message ? If yes, what is the error message -word for word verbatim OR upload an image file showing the error message.

Do you only have one gmail account or are there others? Do you only have a download issue with gmail or others ? Do you have an issue sending?

Is Thunderbird in online or offline mode ? Look bottom left in Status bar - the icon will show whether online or offline.