You can't serve public files from anywhere inside the src directory unfortunately.
You would need to add these files to something like:
js/questionthreads/vendor/fonticonpicker/jquery.fonticonpicker.min.js
.
You would then use the XF JS loader rather than your current approach. If the file is in the above location it would be as follows:
HTML:
<xf:js src="questionthreads/vendor/fonticonpicker/jquery.fonticonpicker.min.js" />
For the CSS, forget about calling them as files. Copy and paste them into new templates and call as follows:
HTML:
<xf:css src="questionthreads_fonticonpicker.css" />
<xf:css src="questionthreads_fonticonpicker_grey.css" />