I've seen both seasons of the TV show Twin Peaks but I've not yet seen the movie Twin Peaks: Fire Walk With Me. Should I make the effort to find a copy of it before the new third season is released?
At the TCA David Lynch makes a clear statement on this Topic. Following some selected articles.
Lynch, the show’s co-creator, said at the TCA presentation that the events of the 1992 prequel movie, Twin Peaks: Fire Walk With Me, will factor into the plot of the new season. (screenrant.con)
[...] when Lynch was asked about the Twin Peaks prequel film, Fire Walk With Me (produced shortly after the series ended), and if it was more of its own entity or not, he gave his one and only hint at what's to come, replying, "I could say it's the story of Laura Palmer's last seven days. Very much important for this." (ign.com)
And as far as I can recall Lynch also said after the final of Twin Peaks Season 2 that the prequel will give all answers. Unfortunately, I couldn't find a quote anymore.
Having seen the first four episodes of the new season, I would say yes - it almost entirely revolves around events in FWWM.
Since I have to give a longer explanation (without spoiling it), one of the minor but important characters from the film
has been mentioned or spoken to by another character in almost every eposide so far. He seems to have a pivotal role. The film, while technically a prequel, also answers a few questions about what happened AFTER series 2 finished.