Technically it was the German company BioNTech, not Pfizer who did the initial mRNA research work for that their joint vaccine.
The history of mRNA based pharmaceuticals is a fascinating one, Statnews have a great article on the topic: https://www.statnews.com/2020/11/10/the-story-of-mrna-how-a-...
In a few years both companies will re-evaluate their relationship. Partnerships can last for years at times. Other times one company is bought. Other times they go their own ways. All are normal and mean nothing, though if you are an investor each has different implications.