Hello You have major coding issues so far in your HTML. You are mixing 2 kinds of code up. Please consult with webmaster and please use World Wide Wed Consortium (W3C) to validate your code. https://validator.w3.org/ If you validate your code it will run in all browsers with no problems. If looking to biz then you need to also make your pages in HTML5 Responsive Code so it fits on all sizes of screens from tablets to phones to big TV's. Also Bootstrap and WP mixed all up. Your errors in here for you https://validator.w3.org/check?uri=http%3A%2F%2Fwww.renzacci.it%2FPagine%2FProdotti%2FLavasecco%2Fdefault.asp%3Flang%3Den&charset=%28detect+automatically%29&doctype=Inline&group=0 Your errors for your CSS pages can be checked here https://jigsaw.w3.org/css-validator/ Your CSS errors are https://jigsaw.w3.org/css-validator/validator?uri=http%3A%2F%2Fwww.renzacci.it%2FPagine%2FProdotti%2FLavasecco%2Fdefault.asp%3Flang%3Den&profile=css3&usermedium=all&warning=1&vextwarning=&lang=en I would think with what I am seeing that you might want to find a HTML5 Responsive Template to work with. There are many free ones https://html5up.net/ and a few good ones where you get many templates for a few dollars such as https://pixelarity.com/ With what you are working with you would also have to pay attention to security updates to the program and then update your pages. If use straight HTML5 it is correct all the time. Yes your pages may work in other browsers but if you want to get a good google ranking then code needs to be perfect. I have pages on the 1st page of a google search. There is no short cuts to doing it right. You might find some one to right some code for Firefox but if looking for google rankings and to sell product..... The above Templates you can adapt your design or information to and the will work. Just keep checking your code and css at W3C. I see your in Italy doing a English page. Have someone that knows English better to check your phrasing, it needs some work. Please let us know if this solved your issue or if need further assistance.