How can I Import contacts from pc?

What do you have the contacts saved to on the PC? What version of Windows are you using, and what version of iTunes? Then, what version of iOS do you have on the iPad? Backing up to the PC or iCloud has nothing to do with syncing/importing contacts from the PC to the iPad.

iTunes cannot sync contacts directly out of Thunderbird, and your iPad cannot sync these files you created via USB. You have a couple of options:

1- Email the .vcf files to yourself and open the file from the Mail app on your iPad. That used to work to import the contacts into the iOS device, not sure if it still does. But even if it does, there is no bi-directional syncing ... changes you make on your iPad or computer don't reach the other device.

2- If you use a web-based email service like Gmail, export your contacts from Thunderbird and import to Gmail. The Gmail account on your iOS device would then import and keep in sync with contacts retained in the Google environment, but not Thunderbird.

3- iCloud can import vCard files, so if you can create a vCard file from Thunderbird you can import that into iCloud.com (you'd have to do this from a computer, not iOS device). Then like #2, iCloud would host your contacts and your iCloud account would sync them to your iOS device. You will have to have an iCloud.com email account, but you don't have to use it for email, it can be used only for Contacts (and Notes, Calender, etc.) without having to use the email portion.

There might be other options that others will chime in with, but there is no way to sync (and automatically keep in sync) Thunderbird contacts with iOS contacts.