To use Wi-Fi with Tehnoetic TET-N300 Wireless USB Adapter for GNU/Linux-libre in Trisquel, I installed the free firmware from the official Trisquel package open-ath9k-htc-firmware. It is included in both Trisquel 7 and Trisquel 6 updates repositories.

sudo apt install open-ath9k-htc-firmware

or, if not running Trisquel and your free deb-based distribution doesn't package the free firmware, download the Trisquel package from a mirror and install the Trisquel package:

sudo dpkg -i open-ath9k-htc-firmware.deb

or, for other free distributions that don't package the free firmware, unpack the deb archive and copy the free firmware to the canonical location:

sudo cp htc_7010.fw /lib/firmware # check if this path exists

or

sudo cp htc_7010.fw /firmware # check if this path exists