The converters the page you are viewing are using java applets (not flash) which hasn't been supported in chrome for some time (https://java.com/en/download/faq/chrome.xml)

FireFox dropped applet support as well (see https://support.mozilla.org/t5/Manage-preferences-and-add-ons/Use-the-Java-plugin-to-view-interactive-content-on-websites/ta-p/3244) and you may have just recently run into this since FireFox 52 shipped earlier this month.

You should likely invest in modifying the site for the more modern web.

Ok thanks, you know how can i see that thing?

Open the page. Right click, Inspect the page. This will bring up dev tools. Find the element on the page that should have the converter you indicate. See the page has:
<applet code="termodinamica.TermodinamicaApplet.class" archive="termodinamica.jar" width="500" height="450" hspace="0" vspace="0" align="middle">
    </applet>

This is a tag loading an applet and not flash.

My question is. is it another form to open that type of archives, like
another page or another navegator?

No the technology the page uses is no longer supported by FireFox or Chrome. I'm not certain if Safari or Internet Explorer might work for you. Your best situation is to find another website that has what you need.