To import EML to Thunderbird you first need to convert them to MBOX format as Thunderbird works with MBOX files. I would suggest the Toolsbaer EML to MBOX Converter for this process.  It quickly import single or multiple EML files into a single MBOX file while protecting all of your email addresses. This converter utility support batch conversion and allows you to import hundreds of EML files simultaneously without experiencing any data loss. It features a simple interface that allows you to preview emails before converting them.  When the MBOX file is prepared you can import it directly into the profile folder of Thunderbird and the emails will appear quickly.