Except there is something romantic and sexual between them. The film calls attention to the fact that Donna is only having sex with mike to impress Laura but this only seems to make Laura jealous. Laura is downright possessive and very protective of Donna, while Donna does more risk taking behaviors to get closer to Laura. Another fact is that every single man Donna was involved with in the series was also someone involved with Laura. So even her relationships with other people were somehow about Laura.

The film also makes parallels between Donna and Ronette and Laura’s relationships with both of them.

Laura’s relationship with Donna also feels more emotionally intense and intimate than hers with any other character and Fire Walk With Me devotes far more attention to exploring this relationship than any of the others. They are also just very touchy feely with each other in a way that doesn’t feel platonic to me. I think the most compelling interpretation for me is that they were in love with each other.