TechnoSlab

Many Firefox users complain about it's unpredictable and high memory usage. In my case, it consumes a small amount of RAM as you can see in the screenshot above but this is not the case with everyone else.
Some users have noticed Firefox's memory usage extend even above 1GB which is not okay because nobody wants their PC to lag just because of a browser.
People should understand that memory usage can be high not only because of buggy program codes but also because of number of tabs currently open, java or video or flash embeds, Javascripts, buggy plugins or extensions etc. While the former can be fixed by sticking to latest release of the program, the later has to be manually sorted out.

Mozilla Firefox - about:memory

You can monitor Firefox's memory usage by going to their about:memory page or about:memory?verbose for a more detailed report. Other than providing detailed analysis of memory usage for tabs and plugins, the page also has a button that helps minimize current memory usage.
To stay away from sudden memory upsurge, stick to latest stable releases of  Mozilla Firefox.
You can know about other significant about pages by going to about:about.