WLMP files actually do not contain video data, they have more or less instructions about how to cut the raw footage:
https://www.online-convert.com/file-format/wlmp

To solve the problem and if you have the raw video files in addition to the WLMP file, you can download the Windows Movie Maker for free and render a video.

Hi, thank you for your report. This is the first time we hear from users having a problem like this. Though very unlikely, we have investigated in the issue and could not reproduce the problem. We have enabled and disabled the plugin several times on several machines and could not see a difference in memory consumption of Firefox. I'm sorry that we can not help here any further.