As I've said before, memory leaks are always epidemic with a voluntary software development organization. No one likes to track down memory leaks, it simply isn't sexy enough. See, gurus don't do memory leaks, they design and prototype and then get some other "flunkie" to clean up the details. Anyone who has worked a development environment knows this.

I have only one tab that is always open (gmail.com) and I have only one plugin enabled (Adobe Flash) and only three extensions installed (NoScript, FlashGot, User Agent Switcher). After surfing for few hours and closing all but one tab, Firefox 4 and 5 uses ~160 MB private memory! And it's not even related to Adobe Flash, because it's running in plugin-container.exe. I have never ever seen so huge memory usage with 3.6 and I don't see any major improvement in 4+ over 3.6. Obviously, while adding useless stuff into browser, Mozilla added loads of memory leaks. I'm unhappy Chrome user now. At least it's not so memory-hungry.

Firefox 5 is no better, nor do we get any better answers. I'm sick of people blaming it on users, extensions and web sites.

"It's the browser, stupid"

https://support.mozilla.com/en-US/questions/837653

The techies are working away at trying to fix memory problems, one fix has resolved several bugs and will be in Firefox 7. Firefox 7 moves to the Aurora channel next week and should be released fully in September.

Update,
Firefox 7 Aurora release Blog - 7th July is saying:

Optimized Memory Use:

   Improved memory management: For many users, memory use is reduced by 30 percent or more, responsiveness is enhanced
   The JavaScript garbage collector runs frequently to free up more memory when Firefox is idle

OK.... I thought FF 5 was the answer but in the last couple of days FF 5 has become unusable, I've now rolled back (again) to 3.6.18. I can't quite understand how 'upgrades' just plain aren't.

I had this same issue. I had to use another browser if I don't want my system to crash. It is already updated to Firefox 5 but still 200MB RAM used

Same problem: sure I have many favorites, etc so normal it gets slower, but it MUST NOT CRASH!!!, please. To prevent it, I must create a new virgin profile. What a shame!!! for a very good browser.

OK, Folks, perhaps this will help. Or at least open up a can of worms...

 Until a few months ago, FF worked great.  So did YouTube, Netflix, most
 websites.  Then.. SOMEthing changed.  Possibilities are:
    product updates: 
       XP, AVAST, Spyware Terminator, Chrome, FireFox,
       Internet Exploder...erm..., Malwarebytes, MS Silverfright,
       Adobe Flash-in-the-pan
    downloads: 
       PROC EXPLORER, PROC MONITOR, other NIRSOFT stuff
         (some nice things there...)
       IOBIT SMART DEFRAG 2

  for starters.  Stuff ran O-K but not great on my KM400A / AMD 1.66Mhz
  underfed (1.15 Gb RAM) vintage 2004 machine.  N.B. NO VIDEO BOARD.
  (oh, forgot. XP SP3 HOME, .NET disabled)
  Watched YouTube,  Netflix, surfed.. and was happy.

     NOW.. VLC crashes instantaneously; Firefox, Chrome, IE start up..
 BUT.. according to PROCEXP memory just keeps a-climbing and a-climbing
 when trying to use Adobe Flash or Silverlight.  On MY pc, anyway. Then
 ka-boom (product crash) or ka-BOOM (system crash).

     On my g/f's PC (1.8GHz AMD, 2MB, Radeon 7000 graphics, XP SP3 PRO
      I left .NET intact) stuff works JUST GREAT. DID do a complete reinstall
      last December, though.

     So, something's eating memory. OR **inhibiting** garbage collection across ALL  browsers.  How does Windoze handle garbage collection?????

    (OR it could be a virus, but MalBytes / AVAST say "neigh!".

     JAVA ???  Eh?

  Same conditions apply even though I now have 2GB ram and a graphics
board: crash, crash, CRASH.

   Anyone got kewl diagnostic tools that might shed some light on this??

   Thanks!

In my experience It's the plugins we use that consume so much ram over time. I now use "Memory Fox" Which reduces my ram consumption from 929MB to under 200MB at all times.

(no I'm not the creator of the plugin) Ha Ha

I hope someone finds this useful. Mozilla team keep up the good work.

If you actually get a crash I suggest

• start your own thread about the crash - bay all means cross linc to or from this thread if you think it helps
• see the tips and suggestions in firefox crashes
  • please ensure you post recent crash ID s in your post
  • consider using the template question linked from the above linked article

As for diagnostics, of crashes I think mainly it is a hard slog and try manually to use safe mode and disable all plugins

• that either
  1. solves the crashing problem, in which case try re-enabling extensions to find the guilty party
  2. or firefox still crashes, in which case your new question may be considered suitable for filing a bug, or provide enough info to find an existing one relating to the problem

For diagnostics of memory problems type in about:memory in the location bar, in the most recent versions of firefox that provides a lot of information, showing what is using up your memory.

Locking this thread. Please follow the suggestions that John99 made in the posting above.