Figma measures your memory usage as a percentage:

A) 60% usage:

Figma will display a yellow warning banner, you cannot dismiss this banner. This file is almost out of browser memory.

B) 75% usage:

Figma will display a red alert banner, you cannot dismiss this banner. Adding content is no longer secure as data may be lost.

C) 100% usage:

If your file reaches 100% of its memory limit, Figma will lock the file and inform you that there is no memory available.