You can try these basic steps in case of issues with web pages:

Reload web page(s) and bypass the cache to refresh possibly outdated or corrupted files.

• Hold down the Shift key and left-click the Reload button
• Press "Ctrl + F5" or press "Ctrl + Shift + R" (Windows,Linux)
• Press "Command + Shift + R" (Mac)

Clear the cache and cookies only from websites that cause problems.

"Clear the Cache":

• Firefox/Tools > Options > Advanced > Network > Cached Web Content: "Clear Now"

"Remove Cookies" from sites causing problems:

• Firefox/Tools > Options > Privacy > Cookies: "Show Cookies"

Start Firefox in Safe Mode to check if one of the extensions (Firefox/Firefox/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem (switch to the DEFAULT theme: Firefox/Firefox/Tools > Add-ons > Appearance).

• Do NOT click the Reset button on the Safe Mode start window.

• https://support.mozilla.org/kb/Safe+Mode
• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

done all of this stuff but problm still not solved...

Doesn't Firefox load any content at all?

• Firefox > Web Developer > Page Source
• Tools > Web Developer > Page Source (Ctrl+U)
• right-click on a web page to open the context menu and select View Page Source

Does this only happen with these websites?

• https://support.mozilla.org/kb/web-pages-dont-load
• https://support.mozilla.org/kb/firefox+cannot+load+websites+but+other+programs+can

• http://kb.mozillazine.org/Error_loading_websites

yes..it happens with some websites like wikipedia and twitter...

same happens with page source...connection time out