Observations of A0V stars are used both for telluric absorption and relative flux calibration. The procedure is outlined in Vacca, Cushing, & Rayner (2003), and implemented in the xtellcor routine of their SpeX pipeline, Spextool. We have ported xtellcor for use with FIRE, and refer users to the relevant xtellcor documentation to see how this procedure is performed in practice.
To call xtellcor from firehose, select the Telluric tab, and then go in the file tree to the exposure you wish to correct. Double click and the xtellcor window should appear with all fields for the input files and B and V magnitudes of the standard correctly populated. If these fields are in error, you can browse to find the right files, and enter in the magnitudes by hand. The input files should be in the redux/Object directory and named Obj_0123.fits with '0123' replaced by the appropriate frame number.
Most Draper catalog telluric stars (i.e. the HD stars) will have both their B and V magnitudes tabulated in the structure. Some hipparcos stars do not have a B magnitude listed, but these can generally be found at the Gemini standards website.
Once the initial telluric calibration frame has been generated for a source visit, only the final correction is required (skipping the kernel calculation and line scaling steps). For subsequent corrections, firehose calls xtellcor_finish to speed up the processing.