Firefox for Android has a new update rolling out that brings several fixes as well as the ability to set it as your default Assistant app. The new update is version 59 for those keeping score at home and has been released to the Play Store. If you have the app installed already, you should see the OTA update for it over the course of the next few days.
The big change in this update is the ability to set Firefox for Android as your default Assistant app. This is done via the Default apps settings on your phone. For the Pixel devices, thats Settings>Apps & notifications>Default apps>Assist & voice input. There you can select Firefox to be your default assistant app. When you do this and you long press your Home button, you will see a search option via Firefox instead of Google Assistant.