Question : We can not access our website, hosted by a 3rd party hosting company, from LAN, but any user from outside can access to the same website.

We've already checked our DNS and firewall, and everything resolves to the ISP's DNS correctly.
For example, we've made sure all our PCs with DNS pointing to our ISP's two DNS servers. Then ran nslookup, everything was resolved.
We can also ping our website name (domain name with or witout www in front of it), and it will translate to our ISP's DNS Web hosting server. However, it doesn't want to load it in the browser.
We also ran trace route, and it went all the way to our ISP as well.
Finally, we connect a laptop directly to our router (outside the firewall), and input our ISP's DNS server IPs to the laptop, it doesn't work either.

Answer : We can not access our website, hosted by a 3rd party hosting company, from LAN, but any user from outside can access to the same website.

Let's assume your server url is www.mywebserve.com.
You also have this topology: LAN > Firewall device > Router/Modem (DSL?) > WAN
Bare with me... This is what we have:
1) You cannot browse http://www.mywebserver.com (Port 80), from your LAN.
2) You can access it an transfer webpages (update) to it, possibly using some FTP client to connect to ftp.mywebserver.com, through Port 21.
2) You can browse other sites (Port 80), from your LAN.
4) Still bypassing the firewall brought no positive results, so firewall is out.
Having this, the only thing that's left is the router and some possible filtering which is being made to your remote host, on port 80... I'm out of ideas...
8(