It is probably the version of Firefox you are using.
Below is a list of DevTest Versions and the Version of Firefox to use based on the Selenium jars we deliver with each version. Hope this helps.
DevTest Firefox Selenium
Version Version we deliver
9.5.1 43, 46 2.53
9.1.0 41 2.48
9.0.0 41 2.46
8.5.0 41 2.46
8.4.0 41 2.46
8.3.0 41 2.46
8.2.0 41 2.45
8.1.0 41 2.45
8.0.2 41 2.45
8.0.1 35 2.44
8.0.0 35 2.44
Selenium Builder 3.0.9 is not compatible with FF 35.