Make sure you are not blocking content.

Start Firefox in Safe Mode {web link}

A small dialog should appear. Click Start In Safe Mode (not Refresh). Is the problem still there?

https://support.mozilla.org/en-US/kb/enhanced-tracking-protection-firefox-desktop

I had already tried that before posting the problem. While posting the problem yesterday, I forgot to mention that the cursor head changes to clickable (i.e. enabled) when I position it on the top border line of the push button object. However, in other parts of the push button area the cursor is disabled (i.e. unclickable). This does not happen while using Chromium. I have also tried on Microsoft Edge in Windows and it works well. I did not try with any other browser.

Many site issues can be caused by corrupt cookies or cache.

Warning ! ! This will log you out of sites you're logged in to. You may also lose any settings for that website.

• Delete cookies to remove the information websites have stored on your computer {web link}

• How to clear the Firefox cache {web link}

I see these buttons disabled as well.

Did you already filled all required fields in the above form ans that might be necessary to unlock these buttons ?

Yes, I did clear the cookies and cache as well as filled all the fields but it still remains unclickable.

OK. I see what is wrong.
If you click the Choose buttons then there is an onclick handler that want to click a hidden (display:none) Browse button ($().trigger('click')). This doesn't seem to work in Firefox when this Browse control is hidden.

You can open the Web Console (Ctrl+Shift+K) and paste this code in the command line and press the Enter key to get these fields in the DOM.

document.querySelectorAll("input#attach,input#attachfitness").forEach(function(c){c.style.display="block";c.style.visibility="hidden";});

• "3-bar" menu button or Tools -> Web Developer
• https://developer.mozilla.org/en-US/Tools/Web_Console

Thanks. That solved the problem for that page in the current session. But, how to permanently fix this issue, so that I don't face the same problem in future in some other web page.

You're welcome.

Each case when a web page isn't compatible with Firefox is different and needs its own recipe to fix it, so you need to look at each case to see if and how this should be fixed.

Ok, then I am closing the case. Thanks.