Gary, a Thos. Jefferson Howell filed a pension application in 1910, listing his unit as the 20th Ms. The filing occurred in Scott Co., Mississippi. Not sure this was your Thomas Jefferson Howell due to differences in unit identification?

I see a "Howell, Chas. T." whose pension application was filed in Jackson County in 1908. He is reported to have been in the 36th Alabama. (No other info on him)

I also see pension applications filed by,

Howell, W.A. in 1924 in Monroe County (5th Mississippi)
Howell, W.H. in 1915 in Panola County (26th Alabama)
Howell, W.J. in 1903 in Monroe County (43rd Mississippi)
Howell, W.P. in 1907 in Desoto County (1st Mississippi Cavalry)
Howell, William in 1916 in Jackson County (12th Alabama)
Howell, William J. in 1920 in Lamar County (6th Mississippi)
Howell, William P. in 1924 in Tate County (1st Mississippi Cavalry)
Howell, William W. in 1904 in Hind County (14th ? ?)
Howell, William H. in 1922, filed by wife, Mary Ann, in Monroe County (43rd Regiment)
Howell, R.C. in 1924, filed by Husband, W.J. Howell in Lamar County (no unit designation)
Howell, R.H. in 1900, filed by Husband, W.B. Howell in Leake County (no unit designation)
Howell, R.H. in 1903, filed in Attala County (33rd Mississippi)
Howell, Susan Jane in 1910, filed by Husband, William W. Howell, in 1910 in Hind County (14th Mississippi Infantry)

That's it, Gary. Hope this helps you find them.