The 22-year-old has yet to make a first team appearance for the Hatters after signing from Histon last summer, having been out injured with a knee injury last term.
However, the left-back has now returned to full fitness, but manager John Still admitted recently he needed a spell on loan to regain his match sharpness.
Fitzsimons links up with the Stortford squad ahead of their visit to St Albans City on Saturday.