You can disable keys with keyconfig:

https://addons.mozilla.org/en-US/thunderbird/addon/dorando-keyconfig/

To install, drag the Download Now button and drop it onto Tools/Add-ons/Extensions in TB.

http://www.chrisramsden.vfast.co.uk/3_How_to_install_Add-ons_in_Thunderbird.html

Thanks. Now I don't need to worry about that J key anymore. But how do I recover everything I had in my inbox?

That depends on what you did to delete from the Inbox. If they were accidentally moved to the Junk folder, drag them back to Inbox.

The problem is trying to find the good ones among thousands of junk emails.

Suggestion: Create a Message Filter to run manually. If you have a load of good email addresses that you store in your address books and those emails may be in Junk.....

• Select the mail account which has the Junk folder
• 'Menu icon' > 'Message Filters' > 'Message Filters'
• click on 'New'
• Give filter appropriate name
• select: 'Manually Run'

If selecting only one address book - one condition

• select 'Match all of following'

If selecting two address books - two conditions or more

• Select 'Match any of following'

• 'FROM' 'is in my address book' 'Personal Address Book'

If you need to add another line, click on the small + sign and enter another condition/ filter parameter on another address book. Perform this action

• 'Move Message To' and select the account Inbox folder
• click on OK

• enable filter and select/highlight that filter

• Run selected filter on: select the Junk folder
• click on 'Run now'