If you have saved web files (.html etc.) go to the relevant folder where you have saved them and right-click on one file. Select open with and you will have a choice of some programmes and also the option "Other". Select other and you will then have the applications list, go to internet and select Mozilla. Be sure that you have ticked the box
"Remember application association for this type of file".
From then on, any file with the same file extention will be opened by Mozilla by default. You may need to repeat this step depending on other file types you have, such as .php etc.
I hope that this helps.