Frequently Asked Questions

Troubles with the runtime

When I launch SlimerJS, nothing append, nothing is displayed in the console

Probably it is launched with an incompatible version of Firefox. If you use a lightweight edition, verify that the environment variable SLIMERJSLAUNCHER contains the path to a Firefox that is compatible with SlimerJS. To verify, launch /path/to/firefox --version. The displayed version should be between MinVersion and MaxVersion indicated into the application.ini file of SlimerJS.

Sometimes SlimerJS hangs, it does not exit, the script is not totally executed

Verify that you have enough free memory on your system.


The rendering of my web site is not really like in my favorite browser. Even in Firefox

A browser is very complex program. The final rendering (the appearance of the web page) depends on many different things:

  • The browser engine (Gecko vs Webkit vs Presto vs Blink vs....).
  • The version of the browser engine (improvements and bug fixes are made in each version of a browser).
  • The library used to draw graphics: depending the operating system the browser is on, it uses a different graphics backend. For example, Firefox uses the Cairo library, which uses DirectX on windows, or other libraries on Linux or MacOs. So there are different algorithm, different implementations, to draw the same thing.
  • The accelerated graphic cards: a browser can use hardware implementation or can use software implementations if the graphic cards has no accelerated graphic chipset. So different algorithm, different implementations: different results.
  • To draw font (if the web page doesn’t use web fonts), it depends on the installed fonts
  • Etc...

So, don’t expect to have exactly the same results between any browser and SlimerJS. Moreover, as SlimerJS is executed on top of Firefox, so At least, the rendering with SlimerJs may be exactly the same as with Firefox (same gecko version, on the same operating system). However you could have some difference in some extrem case (high zooming for example) and with different operating system (even it is the same gecko version).

You can speak with us about rendering issues, on the mailing list.

I don’t see flash content when I take a screenshot with render()

This is because of a bug in Gecko (which was resolved as WONTFIX). However it can work in some cases, when the <object> element contains <param name="wmode" value="transparent">.

When I take a screenshot, the SlimerJS logo appears

First, you can try to give the focus to the window you want to capture :

page.evaluate(function () {

It may not work, especially when you use xvfb and there is no window manager. So a solution is to execute xvfb with a script that launches a window manager (like fluxbox for example), then that launches Slimerjs.

PDF Rendering

On linux, PDF rendering hangs SlimerJS

Verify that Cups is installed and is running. The environment variable CUPS_SERVER can also prevents to produce PDF. It seems that removing it solves the issue sometimes.